Paddick joined the Metropolitan Police Service in 1976. Rising through the ranks, he was appointed the officer in charge of the Criminal Investigation Department (CID) at Notting Hill in 1995, then returned to New Scotland Yard, first as Superintendent of the Personnel Department in 1996 and then as Chief Superintendent in 1997. In December 2000 he was appointed Police Commander for the London Borough of Lambeth where he worked until 2002.
In the latter capacity, Paddick attracted controversy by instructing his police officers not to arrest or charge people found with cannabis so that they could focus on crimes that were affecting the quality of life in the borough to a greater extent. The situation worsened when The Mail on Sunday published a false story by Paddick's former partner that, among other things, he had used cannabis and was an anarchist. In late 2002 the Crown Prosecution Service decided that no charges would be brought against him, and in December 2003 Paddick and the Mail on Sunday settled legal proceedings brought by him, with the newspaper accepting that the story was false, apologising, and paying damages.
In April 2005 Paddick took over management of Territorial Policing across all 32 London Boroughs. During the investigation by the Independent Police Complaints Commission (IPCC) into the wrongful shooting of Jean Charles de Menezes at Stockwell Tube station on 22 July 2005, Paddick stated that a member of the Commissioner of the Metropolitan Police Sir Ian Blair's private office team had believed the wrong man had been targeted just six hours after the shooting. This allegation was contradicted by New Scotland Yard. On 28 March 2006, Paddick accepted a statement from the Metropolitan Police that it "did not intend to imply" a senior officer had misled the probe into the shooting and that "any misunderstanding is regretted".
However, following the disagreement, Paddick was assigned the position of group director of information management at New Scotland Yard, which he considered a "non-job". He came to accept that his police career was over, and retired from the police force on 31 May 2007. On 13 November 2007 it was announced that Paddick had been selected as the Liberal Democrat candidate for Mayor of London in the mayoral elections to be held on 1 May 2008. He came third behind Boris Johnson and Ken Livingstone, with 9.8% of first preference votes.
Paddick returned to New Scotland Yard, first as Superintendent of the Personnel Department in 1996 and then as Chief Superintendent in 1997. He then served as Borough Commander for Merton for two and a half years (1997–1999), responsible for policing Wimbledon, Mitcham and Merton. After attending the Police Staff College in 2000, in December 2000 he was appointed Police Commander for the London Borough of Lambeth where he worked until 2002, fulfilling his ambition of becoming head of policing in Brixton. At Lambeth he had direct responsibility for 940 police officers and 230 support staff, and an annual budget of £37 million.
After a stint in the Specialist Crime Directorate, which deals with serious cases such as murder, kidnap and fraud, Paddick returned to borough-based policing, overseeing the northwest London boroughs of Barnet, Brent, Camden, Ealing, Hammersmith and Fulham, Harrow, Hillingdon, and Islington between 2002 and 2003. He was also responsible for youth and community issues including Police Community Support Officers and police volunteers across the whole of London.
In November 2003 Paddick was promoted to Deputy Assistant Commissioner, and in April 2005 he took over management of Territorial Policing across all 32 London Boroughs, with responsibility for 20,000 police officers and support staff. He was accountable for reducing "volume crime" in London (all offences up to and including rape in terms of seriousness) and increasing the number of offenders brought to justice. He was the national lead for the police service on disability and mental health issues for a year and a half. He was also in the media spotlight as the senior Metropolitan Police Service spokesman for the funeral of Diana, Princess of Wales and after the 7 July 2005 London bombings.
Following a widely-publicised disagreement with Sir Ian Blair, the Commissioner of the Metropolitan Police, over the wrongful shooting of Jean Charles de Menezes at Stockwell Tube station on 22 July 2005, Paddick was assigned the position of group director of information management, which he considered a "non-job". Claiming that the Home Office had intervened for political reasons to ensure that Blair would not have to resign over the incident as it had occurred in the aftermath of 21 July 2005 London bombings, Paddick says he came to accept that his police career was over and that he would never achieve his goal of becoming a chief constable.
Paddick retired from the police force on 31 May 2007, and is currently a Visiting Fellow of Ashridge Business School near Berkhamsted in Hertfordshire, where he lectures on leadership, change management and diversity.
At around this time, the national press also began coverage of the pilot cannabis programme in Brixton where officers were instructed not to arrest or charge people who were found to be in possession of cannabis. They were instead to issue on-the-spot warnings and confiscate the drugs. Although Paddick is credited with the idea, the pilot programme was sanctioned by the Commissioner of Police of the Metropolis, Sir John Stevens.
Paddick asserts that he implemented the policy because he wanted his officers to deal with cannabis quickly and informally so that they could concentrate on heroin and crack cocaine offences, and street robbery and burglary, which were affecting the quality of life in Lambeth to a greater extent. "[P]olice officers said they weren't prepared any longer to drop cannabis down the drain because one of their colleagues had been arrested by internal investigators apparently for doing just that. They were going to arrest everybody they found even with the smallest amount of cannabis. I couldn't have allowed that to happen. They would have been doing nothing else."
Concentrating on Paddick, it accused him of being soft on drugs. The policy became a source of public controversy and debate. The situation worsened when The Mail on Sunday published a false story by his former partner, James Renolleau, that Commander Paddick had used cannabis himself. They also highlighted his homosexuality as a matter of concern, and claimed he was an anarchist based on a comment he had made on Urban75 in January 2002. In fact, he had written that "the concept of anarchy has always appealed to me", but that he was "not sure everyone would behave well if there were no laws and no system.
Following the controversy, Paddick was transferred to an intelligence position and the allegations were investigated by the Crown Prosecution Service. His transfer led to public rallies in Lambeth in his support. The CPS decided in late 2002 that no charges would be brought. In November 2003, Paddick was promoted to Deputy Assistant Commissioner for Territorial Policing. In a December 2003 out-of-court settlement, the Mail on Sunday accepted that their story was false, apologised, and paid damages.
The situation did, however, encourage the British government to re-evaluate its policies with regard to drugs, and a change in the law to decriminalise cannabis from a class B to a class C drug was suggested. Subsequently the law was altered in February 2004.
In September 2007 Paddick commented: "I always felt like a fish out of water in the police, not just on gay issues but generally. I was a very counter-cultural senior officer. I'm very non-hierarchical and got into trouble for insisting on people calling me by my first name. I was trying to effect the most difficult change there is in an organisation, which is a change of culture – to try to make it more liberal, more understanding of difference."
After the 21 July 2005 London bombings and the subsequent police shooting of Jean Charles de Menezes on 22 July 2005 at the Stockwell Tube station in London, Paddick met with Stockwell community leaders. Kate Hoey, Member of Parliament for Stockwell, had requested the meeting to help reassure local residents.
Paddick was quoted by the BBC as stating, "It was a very calm meeting. People were very understanding of the circumstances that these officers found themselves in. We expressed our sympathy to the family of the person who was tragically killed. It was a very measured meeting, but a very positive meeting." He would not promise that a similar tragedy would never happen again, but he stated it was not likely.
In March 2006 it was revealed that Paddick had consulted libel lawyers in connection with statements issued by Scotland Yard. In verbal and written statements to the Independent Police Complaints Commission (IPCC) on their investigations into the shooting of Jean Charles de Menezes, he had stated that a member of the Commissioner of the Metropolitan Police Sir Ian Blair's private office team believed the wrong man had been targeted just six hours after the shooting. This was contrary to statements made at the time.
When this allegation became public following an unauthorised disclosure, Scotland Yard issued a statement claiming that the officer alleged to have believed this [Paddick] "has categorically denied this in his interview with, and statement to, the IPCC investigators". The statement continued that they "were satisfied that whatever the reasons for this suggestion being made, it is simply not true". Paddick's interpretation of this statement was that it accused him of lying.
On 28 March 2006, Paddick accepted a statement from the Metropolitan Police that it "did not intend to imply" a senior officer had misled the probe into the shooting of Jean Charles de Menezes. In a statement, the Metropolitan Police said "any misunderstanding is regretted" and that Paddick had accepted its "clarification" and considered the matter closed.
In a lengthy interview with Paddick published in The Daily Telegraph on 17 November 2007 detailing his thinking on becoming London Mayor, he made a revealing comment possibly connected with difficulties that have arisen because of the de Menezes killing: "Policing is a dangerous job, we should trust the professional judgement of officers on the front line. We shouldn't prosecute them or their bosses if they decide to put their lives on the line for the public. In addition, on 26 November 2007 in The Independent Paddick was asked if Commissioner Ian Blair should have resigned over the de Menezes case. His response was: "Yes. The public allows the police to use force on the understanding that someone will be held to account if something goes wrong. An innocent man was killed by the police and in the absence of any individual officer being held to account, the person at the top should take responsibility.
In August 2006 it was reported by The Observer's 'Pendennis' column that Paddick was considering seeking selection as the Liberal Democrats' candidate for Mayor of London in the mayoral elections to be held in 2008. This was confirmed a year later by the BBC. He received an endorsement from Lynne Featherstone MP, a former member of the London Assembly, on her blog.
On 2 September 2007, Paddick informed Pink News of some of his priorities if elected mayor. On illegal drugs, he reiterated that they are "dangerous and harmful and it is better if people live without them", but that he had a "realistic approach" to enforcement and would not be afraid of adopting a "radical solution if it's fully thought through, if it's workable, affordable and gets the right result". He promised to regulate the timings of London's buses and to apply the London congestion charge in a more sophisticated manner that differentiated wealthy chauffeur-driven chief executives from delivery drivers. As regards the 2012 Summer Olympics to be held in London, he pledged to ensure that "the best possible show" would be staged with "value for money for every single pound spent". In addition, in a leaflet released on his campaign website on 10 September, he pledged to "put more police officers, fully trained, equipped and with the necessary powers out on the street, dealing with violent gun and knife crime", to take the lead in tackling climate change and minimizing the impact on the environment by powering the London Underground with renewable energy, and to provide the right environment for business without interfering in businesses.
On 13 November 2007 it was announced that Paddick had been selected as the Liberal Democrat candidate for Mayor of London. He won 73% of the first preference votes during the selection contest. Paddick joined, among others, Ken Livingstone as the Labour candidate and Conservative Party candidate Boris Johnson. However, a poll conducted by The Guardian and ICM published on 3 April 2008 found that Paddick was only the first choice of 10% of voters. He therefore had little chance of winning, as 42% supported Johnson and 41% Livingstone. The poll was borne out by the results of the elections on 1 May 2008. On 3 May it was announced that Boris Johnson had been elected. Paddick was in third place behind Ken Livingstone, with 9.8% (236,685 out of 2,415,958) of the first preference votes.
In 2007, Paddick was ranked number 83 in the annual Pink List of 100 influential gay and lesbian people in Britain published by The Independent on Sunday.