Forgan entered local politics as a member of Glasgow council after seeing active service in the war. Initially a member of the Independent Labour Party, he was elected to Parliament for West Renfrewshire in the 1929 general election. An early triumph saw him secure the installation of a ventilation system into the House of Commons, although after this he became a fairly marginal figure. Forgan was one of the signatories of the 'Mosley Memorandum' which outlined his political vision and he followed Mosley into the New Party when it was set up soon afterwards. He had officially left the Labour Party on 24 February 1931 and, sometime that year, co-authored with Adam Marshall DistonThe New Party and the ILP. He was appointed to a council for policy and strategy formation that was set up to decide the running of the party and also acted as Chief Whip during the New Party's brief run in Parliament. At the 1931 general election, Forgan polled 1,304 votes in West Renfrewshire in what represented one of the better results for the New Party in a disappointing election. A close friend of Oswald Mosley, Forgan was godfather to his son Michael.
With Mosley having embraced fascism Forgan followed his lead and on Mosley's behalf led unsuccessful talks with the British Fascists, aimed at having that movement taken over by the New Party. Forgan joined Mosley's British Union of Fascists and was initially Director of Organisation. This administrative role did not prove suitable and soon he became an important background figure, arranging private functions with leading businessmen in an attempt to secure support for Mosley and organising the January Club to this end. Forgan was keen to stress that the BUF had no ban on Jews despite the activities of Adolf Hitler. Indeed, Forgan attempted to court influential Jews, such as Liberal MP Harry Nathan and Sir Philip Magnus-Allcroft, 2nd Baronet, through the January Club and even held meetings with the leaders of the Board of Deputies of British Jews. Forgan was also keen to keep the BUF aloof from rival far right groups such as the Imperial Fascist League as he felt it was essential to avoid making the BUF seem too foreign in ideological terms. As a result of his work, Forgan was promoted to deputy leader. He held that position until 1934 when he left the BUF because of its drift towards anti-Semitism. Robert Skidelsky has argued that Forgan's conversion to fascism had always been at best half-hearted and had more to do with his personal loyalty to Mosley, something that was largely gone by that point. Forgan in particular disliked the growing influence of William Joyce, a staunch anti-Semite, who replaced Wilfred Risdon, a colleague of Forgan's from the ILP and the New Party, as Director of Propaganda the same year. He took no further role in politics.
