On a more positive note, his goalkeeping helped Ipswich beat United (then in the early years of their current dominance of English football) on two occasions in the first three seasons of the Premier League - a 2-1 win at Portman Road in February 1993 and a 3-2 victory there in September 1994 - as well as holding them to draws at Old Trafford in August 1992 and November 1993 - on the latter occasion he ensured that Ipswich were one of just three sides who visited United in the league without conceding a goal that season. He was voted Goalkeeper of the Month in the Premier League for October 1994, and around this time an offer of around £2million from an unnamed club for Forrest's services was rejected by Ipswich. He remained loyal to the club despite their relegation at the end of that season, and stayed there for another two seasons.
Forrest went on loan to Chelsea from Ipswich in 1997, playing in the same team with the likes of Gianluca Vialli and Gianfranco Zola under the guidance of manager Ruud Gullit. A permanent move to Chelsea was turned down by the Ipswich Board of Directors.
The goalkeeper ended up moving to West Ham United in the summer of 1997 for £500,000. Forrest played a number of first-team games in the FA Premier League in the late 1990s and early 2000s with the east Londoners, remaining at Boleyn Ground until his retirement from football in 2002. On 1 April 2000 he was again the goalkeeper for a big defeat by Manchester United. Although West Ham scored first, the game finished 7-1 to Manchester United.
Forrest played at the 1987 FIFA U-20 World Cup and made his debut for the Canada senior team in a May 1988 friendly match against Chile. He earned 56 caps (excluding the May 1990 North American Championship match against the USA), the most of any goalkeeper in the national team's history tied with Pat Onstad and earned the most clean sheets in the country's history before retiring from international football in 2002. The highlight of his international career was undoubtedly helping Canada win the 2000 CONCACAF Gold Cup, in which Forrest was named tournament MVP and Most Valuable Goalkeeper for allowing a mere three goals and stopping two penalties in his side's five games. Also during his international career Canada had secured a 1–1 draw against Brazil only a month prior to the World Cup in U.S.A. in 1994. Forrest had further success against Brazil in 2001, recording a clean sheet in a 0–0 draw in the 2001 Confederations Cup in Japan. In 2012, Forrest was voted by the fans into the Canadian best XI in the CSA's first 100 years.
Forrest was forced to retire from professional soccer due to complications from acquiring and recovering from testicular cancer. He began working as a sports analyst with Rogers Sportsnet (now Sportsnet) upon his retirement from playing. Forrest is considered by some to be one of the best players ever for Canada and continues to play a major role in promoting the game in the country, appearing frequently as a broadcaster for the men's national team and Toronto FC on Sportsnet as well as Sportsnet's Soccer Central Matchday.