McLennan joined Aberdeen at the age of eight, and turned professional aged 16 in December 2015. He made his debut as a substitute away to St Johnstone on 22 April 2016.
On 16 December 2016, McLennan joined Scottish League One side Brechin City on a one-month emergency loan deal, playing 3 times and scoring 1 goal. He returned to Brechin for a second loan spell on 30 August 2017.