After graduating with a lower second class degree in 1941, he joined the war effort as a commissioned officer in the Royal Signals. He then served with the Guards Armoured Divisional Signals, both domestically and in North-West Europe, until the end of the war in 1945. He served in the military government of Schleswig-Holstein, and was demobbed as a lieutenant-colonel aged only 25.
He married his wife, Mary (later Dame Mary Donaldson), in 1945, having met her at Middlesex Hospital where she was serving as a nurse. She later became the first woman to be a member of the City of London Court of Common Council, the first female alderman, the first female sheriff and, finally, in 1983, the first female Lord Mayor of London. Together, they had two daughters and a son. His wife predeceased him in October 2003.
He became the first (and last) President of the abortive National Industrial Relations Court (NIRC, also known as the Industrial Relations Tribunal) from its formation by Ted Heath's Conservative government in 1971 under the Industrial Relations Act 1971 until it was abolished in 1974. The trades unions doubted his independence, pointing to his Tory inclinations in his youth, and nicknamed him "Black Jack", and 181 Members of Parliament (MPs) signed a House of Commons motion calling for his dismissal. However, lawyers who appeared before him considered that he acted fairly throughout. The Court was abolished when Labour returned to power in 1974, and Donaldson returned to the bench at the Commercial Court in 1974, where he remained for five years. Many observers consider that Labour Party politicians passed him over for promotion to higher judicial office while they remained in power.
Two months after Margaret Thatcher was elected in 1979, he became a Lord Justice of Appeal, and was thus was automatically appointed to the Privy Council. He replaced Lord Denning as Master of the Rolls in 1982, becoming the presiding officer of the civil division of the Court of Appeal of England and Wales, where he pushed forward modernisation efforts, including the introduction of skeleton arguments in civil appeals, judgments being "handing down" rather than read, and enhanced case management. Continuing his antagonistic approach in the labour tribunals to industrial relations, Donaldson decided in O'Kelly v. Trusthouse Forte plc  ICR 728, Donaldsdon's early reforms would later be overtaken by the Civil Procedure Rules introduced by a later Master of the Rolls, Lord Woolf. In 1988 he was elevated to a life peerage as Baron Donaldson of Lymington, of Lymington in the County of Hampshire.
In his various roles, Donaldson was involved in many high-profile cases from the 1970s onwards. He presided over the trials of the Guildford Four in 1975 and the Maguire Seven in 1976, and was later criticised in Sir John May's interim report of his inquiry into the miscarriages of justice; he refused to prevent newspapers from publishing the memoirs of Peter Wright in 1988; and he ruled in 1991 that the then Home Secretary, Kenneth Baker was in contempt of court over an extradition case, in which a man was deported to Zaire while the case was still pending, contrary to a court order.
In the 2000-2001 session of Parliament, he presented a Private Member's Bill in the House of Lords (the Parliament Acts (Amendment) Bill), which would have had the effect of confirming the legitimacy of the Parliament Act 1949 to address concerns raised by legal academics as to whether the use of the Act was valid. The Bill was not passed, and Donaldson supported the legal action by the Countryside Alliance to overturn the Hunting Act 2004, which was passed under the provisions of the Parliament Acts.
MAIL'S COURT WIN: Sunken Donuts; EXCLUSIVE Sheriff Throws out Crime Inc Millionaire's Bizarre Court Bid to Gag the Sunday Mail
Sep 19, 2004; Byline: NORMAN SILVESTER CRIME Inc. boss Frankie 'Donuts' Donaldson has lost a bizarre court bid to gag the Sunday Mail. Paranoid...