(CNN) -- Premier League strugglers Portsmouth eased their relegation fears with a 2-0 win over Manchester City at Fratton Park on Saturday.
Johnson is hugged by caretaker manager Paul Hart after scoring Portsmouth's first goal.
Second half goals from Glen Johnson and Hermain Hreidarsson secured a valuable three points for Pompey who sacked manager Tony Adams on Monday after a series of poor results.
It was City's seventh defeat away from home in the Premier League this season and Brazil international star Robinho, so impressive in the 2-0 midweek win over Italy, was substituted with 25 minutes remaining.
Portsmouth goalkeeper David James was making a new record of 536 Premier League appearances and celebrated with a clean sheet.
His superb save from substitute Ched Evans in the 80th minute capped a fine performance and came at a vital time with Portsmouth just 1-0 ahead.
Caretaker manager Paul Hart and his assistant Brian Kidd would have been aware that the reverse fixture had led to a 6-0 thrashing at Eastlands back in September, but it was their team who had the better opportunities in the first half.
Striker David Nugent missed two good chances, the second a clear header after fine cross by Glen Johnson.
Portsmouth continued to press after the break and Nugent missed another opportunity before the excellent Johnson scored the opener in the 70th minute.
His first shot was saved by Shay Given, but it came straight back to him and he smashed home the rebound.
Hreidarsson sealed victory in the 85th minute as he headed home on-loan Jermaine Pennant's cross.