Longmuir, the former chief executive of the Scottish Football League, lost out to SPL counterpart Neil Doncaster last month in the race to head up the newly-formed organisation.
The duo were the only candidates vying for the chance to take the game forward following the merger of Scotland's two professional leagues.
Longmuir had been in charge of the lower leagues north of the border since 2007, having previously worked in the alcohol industry.
In a statement, the SPFL said: "The SPFL can confirm that following the decision to appoint Neil Doncaster to the role of CEO in the SPFL, David Longmuir's employment has come to an end."
The new SPFL, which retains the 12-10-10-10 set up, was given the green light by Scotland's 42 senior clubs following a number of meetings at Hampden last month.