The former Celtic defender, who joined United from Partick Thistle in January of this year, lead his side to two wins and a draw from three league matches and capped off the month with a stunning 5-2 Scottish Cup victory over Kilmarnock.
The unbeaten run, which saw United score 14 goals with just four against, lifted them to fourth place in the SPFL table just five points behind second-placed Motherwell.
The current form is in stark contrast to the start of the season when McNamara's side were kept off bottom spot by Hearts' 15-point deduction after taking just two points from their opening three matches.
McNamara's award completes a memorable double for the Tannadice club after full-back Andy Robertson was named SPLF Player of the Month earlier in the day.
The 19-year-old, who won the young player of the month award in September and has recently committed to a long-term deal, scored in the 4-0 defeat of Motherwell at Fir Park and in the 4-1 home victory over Partick Thistle before grabbing a double in the cup win over Kilmarnock.