He initially played Australian rules football for the local team the Aces, but opted to pursue the sport of weightlifting. In 1989 the Nauru Weightlifting Federation (NWF) was founded, primarily to give Stephen, the sole top-class weightlifter in Nauru at the time, the opportunity to compete internationally.
In 1992 he took part in his first Olympic Games in Barcelona. Since Nauru had no Olympic Committee at the time, he successfully applied for Samoan citizenship and was allowed to compete for Samoa. In 1993 the committee was founded and Stephen was able to represent Nauru in the 1996 Summer Olympics in Atlanta and the 2000 Summer Olympics in Sydney.
When the Olympic torch was carried to Sydney, Stephen had the honour of being a torch bearer during its stopover in Nauru.
It was at the Commonwealth Games where most of his successes came: In the 1990 Commonwealth Games he surprisingly won a gold medal in the Snatch in the 60kg class. In the 1994 Commonwealth Games he won three gold medals in the 59kg class and in the 1998 Commonwealth Games in the 62kg class he collected three more golds. In his last Games, the 2002 Commonwealth Games in Manchester he won three silvers in the 62kg class.
His greatest triumph came at the 1999 World Championship in Athens, where he was runner up in the clean and jerk in the 62kg class.
In March 2008, it was reported that Stephen had been appointed to the presidency of the Oceania Weightlifting Federation, the Pacific region's official continental weightlifting body.
During the presidency of René Harris Stephen was Education and Finance Minister of Nauru from 8 August 2003 until 22 June 2004, a post he had to relinquish when Ludwig Scotty became the new president. In October 2004 he was re-elected to parliament.
After Nauru joined the International Whaling Commission (IWC) on 15 June 2005, Stephen was nominated as a delegate for Nauru. He represented Nauru at the IWC-Congress in June 2005 in the South Korean city of Ulsan.
Stephen was re-elected to parliament in the August 2007 parliamentary election. He unsuccessfully stood as a presidential candidate in the vote held in parliament on August 28, in which Scotty was re-elected.
In the initial period of his Administration, Stephen moved to continue the practice, commenced by former President Ludwig Scotty, of appointing a separate Foreign Affairs Minister, when Dr. Kieren Keke (a cousin of Stephen) was installed in that post in December 2007. Previously, each President of Nauru had concurrently acted as his own Foreign Affairs Minister, although this was customary rather than a constitutional requirement. Stephen also appointed Frederick Pitcher as Finance Minister, and his Administration inherited the austerity meausures associated with the outgoing Scotty Administration.
Regarding constitutional affairs, however, one of the major issues facing the new Stephen Administration was the process of constitutional revision consultations, started by former President Ludwig Scotty. These centred mainly on proposals to elect the President of Nauru by direct, popular election, rather than indirectly by the Parliament of Nauru, and which would thus restrict somewhat the frequent recourse to the vote of no confidence, which has been a feature of Nauru's political life for many years. Since Marcus Stephen came to office in circumstances involving the overturning of the previous government's Parliamentary majority in just such a manner, it remained to be seen what the Stephen Administration's formal position and practice on this issue would be.
At a personal level, the appointment of Marcus Stephen as President of Nauru at the age of 38, together with a youthful ministerial team, marked somewhat of a generational shift from some of the political figures who have dominated Nauruan politics in recent years; e.g., he was nearly 40 years younger than Derog Gioura, who served as President of Nauru in 2003. However, the appointment of youthful heads of state in the Republic of Nauru is by no means unknown; Bernard Dowiyogo assumed the office of President of Nauru at the even younger age of 30 in 1976.
As a maritime nation, Nauru's membership of the International Whaling Commission put the Stephen Administration in the spotlight, particularly in view of the IWC meeting in 2008, to be held in June, 2008, at Santiago, Chile. At the United Nations under previous government leadership, Nauru's Permanent Representative, Marlene Moses has voiced a pragmatic approach to issues of stock conservation, and, particularly given pressures to preserve the livelihood of Nauruans engaged in the tuna fishing industry, the Stephen Administration is likely to find it challenging to alter the stance taken at the IWC by previous governments; but the whaling file is one with which President Stephen has been very familiar, given his past appointment as a delegate to the IWC in 2005.
In March 2008 moves in the Parliament of Nauru to unseat the Administration of Marcus Stephen by means of a vote of no confidence were thwarted by the resignation of the Speaker, Riddell Akua. Unrest on the island which involved threats to export trade and the torching of a police station were events which occurred shortly prior to Parliamentary moves to remove President Stephen and his Administration from office.
At the end of the first three months of Stephen's Presidency there was thus widespread unrest in the country.
The Stephen Administration, in response, denied the claim of a coup d'état, stating that they were awaiting a ruling from the Supreme Court on the issue.
The crisis continued into early April 2008, with Adeang stating that he would consider the Supreme Court's ruling as "just an opinion", and Keke responding that the Supreme Court, not the Speaker, had the jurisdiction to determine a member of Parliament's eligibility.
Relations between the Stephen Administration and Adeang remained under severe strain, however, and the Administration's ministers continued to exercise executive powers without the support of an absolute parliamentary makority.