One day, MacTavish was invited by his cousin, a member of the 23 Regiment of the Special Air Service, to see how it was like to be in the British Army. Afterwards, MacTavish often visited his cousin on weekends. When he was 16, he tried several times to enroll in the SAS and while he lied about his age, he was caught every time.

After his 18th birthday, MacTavish officially joined selection for the 22 Regiment, an elite squadron specialized in covert reconnaissance, counter-terrorism, and hostage situations.