He cuts inside and shoots, rattling a deflected shot into the side netting and winning a corner.
Philip Zinckernagel got on the end of a fabulous run and cross from Johnson, drawing Bernd Leno into a strong save with a driven effort.
The League One side are in front against Liverpool.
Arsenal had plenty of the ball but Forest were never concerned during an instantly forgettable first half - which would have suited the hosts, who restarted quickly after the break.
Arsenal - currently sitting fourth in the Premier League - were knocked out by Southampton in the fourth round last year and will be eyeing another deep run this season having also reached the semi-finals of the Carabao Cup.
Live U-TV odds are viewable on SofaScore's section.