He was born on 9 February 1883 at 105 St. Leonard Street, Edinburgh, to John and Elizabeth (née Stewart) McLennan, the eighth of their nine children (one of whom died in infancy).

He made rapid progression, winning the Amateur National Championship at the age of nine, and was invited by Queen Victoria to play for her at Balmoral Castle.

[4][2] His father enlisted him in the Gordon Highlanders in October 1899 in order to prevent him from joining the Merchant Navy, and he became Pipe major of the 1st Battalion in 1905, one of the youngest ever in the British Army.

[5] McLennan was posted at the depot in Aberdeen until 1918, when he was sent to the Western Front to succeed Pipe Major Tom Henderson who had been killed.

20,000 people lined the route of the procession to Aberdeen station at his funeral on 4 June, before he was interred at Newington Cemetery in Edinburgh.