King Tupou VI approved the appointment.
The Australian police commander takes up the position from former New Zealand police superintendent Stephen Caldwell, who finished up in 2020.
The appointment of a new commissioner was delayed by the pandemic, and the position was filled in the interim by Tongan army brigadier, Lord Fielakepa.
Mr MacLennan has extensive experience, working for the Australian Police since 1981.
He was the Airport Police Commander at Brisbane Airport as well as a Detective Superintendent with the Australian Federal Police.