Brennan played at the 1993 FIFA U-17 World Championship in Japan, in a team alongside Paul Stalteri and Jason Bent. He then made his senior debut for Canada in an April 1999 friendly match against Northern Ireland and went on to earn a total of 49 caps, scoring 6 goals. He has represented Canada in 10 FIFA World Cup qualification matches. He has played for Canada at the Confederations Cup 2001 and played against Brazil, Cameroon and hosts Japan as well he competed at the 2005 CONCACAF Gold Cup. Brennan won a gold medal in the 2000 CONCACAF Gold Cup defeating Colombia and bronze medal in the 2002 CONCACAF Gold Cup defeating South Korea.