The Glasgow club are looking to increase the gap between them and the relegation places with a win.
Doolan should have added a second after eight minutes, rounding St Johnstone keeper Alan Mannus but shooting wide of the open goal.
Partick's lead was doubled a minute later when central defender Conrad Balatoni headed into the roof of the net.
With the Saints defence at sixes and sevens in the opening stages of the match, Ryan Stevenson's attempt to add a third was stopped by Mannus.
Partick continued to dominate and Stevenson smashed a volley against the left upright in the 40th minute.
Michael O'Halloran tried to pull a goal back for the Saints before half time, but his shot went over the bar.