Lorimer was born in Edinburgh, the son of James Lorimer, who was Regius Professor of Public Law at Edinburgh University from 1862 to 1890. He was educated at Edinburgh Academy and later at Edinburgh University. He was part of a gifted family, being the younger brother of painter John Henry Lorimer, and father to the sculptor Hew Lorimer. In 1878 the Lorimer family acquired the lease of Kellie Castle in Fife and began its restoration for use as a holiday home.
Lorimer began his architectural career working for Sir Robert Rowand Anderson, and went on to form his own practice in 1893. He was influenced by Scottish domestic architecture of the 16th and 17th centuries and the Scots Baronial style of Kellie Castle where he had spent much time as a young man. Early in his career, Lorimer became influenced by the ideas of William Morris, and went on to become a committed exponent of the Arts and Crafts style of architecture. He assembled a collaboration of artists and craftsmen and, collectively, they exhibited furniture at Arts and Crafts exhibitions in London. In 1896 he was elected to the Art Workers Guild.
Lorimer designed a series of cottages in the Arts and Crafts style in the Colinton area of Edinburgh, the so-called "Colinton Cottages". Constructed using traditional methods and materials, each cottage included a garden layout and interior design, including furniture, in keeping with the Arts and Crafts concept. By 1900, eight cottages had been built and four others were under construction. The decline in popularity of the Arts and Crafts movement from 1900 saw the direction of Lorimer's work change, and he undertook several large scale country house commissions, mainly designed in the Scots Baronial style of which Ardkinglas is a particularly notable example.
The outbreak of World War I restricted the demand for large new houses and his attention shifted to restoration projects. He already had a reputation as one of Scotland's leading restoration architects following the restoration of Earlshall in 1899 and Hill of Tarvit in 1905, both in Fife. He went on to carry out significant alteration and restoration works at Lennoxlove House in East Lothian and Dunrobin Castle in Sutherland.
Although much of his work, and reputation, was in the sphere of domestic architecture, Lorimer also carried out significant public works. Principal amongst these include his design for the new chapel for the Knights of the Thistle in St Giles Cathedral, Edinburgh in 1911. He received a knighthood for his efforts and went on to gain the commission for the Scottish National War Memorial at Edinburgh Castle in 1919, subsequently opened by the Prince of Wales in 1927.
Lorimer became President of the professional body in Scotland, the Incorporation of Architects in Scotland, and it was during his tenure in office that the body received its second Royal Charter, permitting use of the term 'Royal' in the title. He died in Edinburgh in 1929.