MacKinnon sought to volunteer for service during the first world war, but was not accepted, probably due to his age. His older brother Roger had enlisted in 1916.

By 1916, he was part of the J.C. Williamson theatre company. The absence of a main cast member in 1918 provided this 'promising youngster' a feature role in The Bing Boys are Here review at Her Majesty's Theatre (The Sydney Morning Herald, 25 March 1918). The following year, he was with the J.C. Williamson Company in Melbourne (The Daily Telegraph, 13 November 1919). He probably continued as a live performer into the 1930s.

During this period, he also acted in several black and white silent films, including The Man from Snowy River (1920) for which he was also assistant director, The Gentleman Bushranger (1921), While the Billy Boils (1921), Silks and Saddles (1921) which was renamed Queen of the Turf in the US, and A Rough Passage (1922).

In 1922, on a bet of £50 with a US visitor, MacKinnon took up the challenge of copying US Locklear's feat of hanging from the under-carriage of an plane at 3000 feet (Smith's Weekly, 25 March 1922).

After this time, MacKinnon spent years as a jackaroo. He worked on station properties handling sheep and cattle, and was a capable horseman (The Australian Women's Weekly, 9 June 1945).

By 1944, he had returned to Sydney and scripted Missing, a film to raise funds for the Victory Loan produced by Ken G. Hall (of Dad and Dave fame) for the Department of Information, focused on prisoners of war in the Pacific (The Sun, 13 April 1944).

MacKinnon subsequently returned to Sydney and became a prolific author of novels and radio plays. He claims to have dictated to two secretaries at once, producing 15,000 words a day and 1.5 million words a year (People, 2 August 1950).

In the mid to late 1940s he was a staff writer for Sydney radio station 2GB, creating a range of syndicated serials such as 'Sporting Blood' (1945-1946), 'Jackeroo Joy' (1946-1947), 'The Woodleys' (1946-1952), 'Kookaburra Stories' (1946-1949), 'My Son, Tom' (1948-1950), 'Silks and Saddles' (1948-1949) and 'The Dark Stranger' (1949-1950). He wrote some 600 scripts a year (The Sun, 9 August 1946).

Up until 1950, MacKinnon wrote many novels for Frank Johnson, particularly about horse racing. Sales of his monthly Sporting Novels exceeded half a million copies yearly. A short-lived magazine in 1950, Bob McKinnon's Racing and Sports Magazine (spelling uncertain), appears to use his reputation for its branding.

MacKinnon was an amateur jockey and his father, Dr. R.R.S. MacKinnon was an honorary surgeon for the Australian Jockey Club (AJC) for more than 30 years (The Sunday Herald, 15 May 1949). He also professes to been an "amateur jockey, shearer's rouseabout, station manager, jackeroo, pioneer aviation barnstormer, and aero acrobatic star (first man to perform wing-walking in Australia)." (The Rogue (Sydney: MacKinnon, 1950))

Around 1950, he formed his own publishing company, Mackinnon Publications. The 'Spotlight on Sport Series' was a effectively a continuation of Johnson's 'Sporting Novels'.