Lomas was banished to the stands by referee Iain Brines during the 1-1 draw with Celtic at Parkhead earlier this month.
The former West Ham captain was angered when Brines allowed Celtic striker Tony Watt back on to the field following treatment in what he felt was a dangerous position.
Lomas apologised in person to Brines after the match, but the SFA disciplinary committee found him guilty of using foul and abusive language towards an official and imposed a six-match ban from the dugout.
But Lomas had also incurred a suspended two-match ban following his dismissal in the clash with Ross County in September.
A statement on St Johnstone website read: "The SFA have handed Steve Lomas an eight-match touchline suspension following his dismissal from the dugout in the match against Celtic on 11th November for foul and abusive language towards a match official.
"This ban consists of six games for the offence committed at Celtic Park plus the suspended two-game ban which was already hanging over him following the game against Ross County in Dingwall in September."
Lomas is facing further punishment after again being sent to the stand towards the end of his side's 1-0 defeat by Hibernian on Wednesday evening.