Mortimer was born in Calton, Glasgow, and played for a variety of local junior clubs in the city before moving to London in 1894 to play for Woolwich Arsenal, being one of many Scottish players at the club at the time.
The next season, Mortimer generally played at left half rather than up front, and thus scored fewer goals; he still bagged nine that season, including a hat-trick in a 7–0 defeat of Crewe Alexandra.
However, at the end of the 1895–96 season he was transferred to Chatham for unknown "political" reasons.
After leaving football he worked as a railway carriage painter and ironmonger.
This biographical article related to association football in Scotland, about a forward born in the 1870s, is a stub.