Along with Baihakki Khaizan, Shahril Ishak and Khairul Amri, Hassan was in the pioneer batch of the National Football Academy in 2000.
Hassan has previously played for S.League clubs Tampines Rovers, Geylang United and Young Lions.
According to an interview with him done in November 2007, he only started playing football competitively when he was 10, captaining his school, May Primary School. He later went on to represent Braddell Secondary School football team as well.
Hassan started out in football as a midfielder, only switching to between the sticks due to an asthma attack he suffered when he was 12. He also quipped that his happiest football memory was that, back in 1999, he scored the winning goal from the halfway line in a game that ended 2-1.
Hassan joined the Eagles from the Singapore U-18 team and quickly cemented his spot in the first team, earning an S.League Young Player of the Year nomination for his confident performances and knack of pulling off blinding saves.
In December 2011, Hassan joined SAFFC, after four years of service as Tampines Rovers' first-choice goalkeeper.
Hassan became the first goalkeeper to win the S.League Player of the Year award in 2014.
Hassan joined Army United F.C. for the 2015 Thai Premier League season. He made his debut in a 1-0 win over Royal Thai Navy F.C.. After a series of games in good form Hassan excelled in a surprising 1-0 away win at Suphanburi F.C., handing the latter side their first home defeat of the 2015 season as well as giving Hassan man-of-the-match award. After the game Suphanburi player Jakkaphan Pornsai said Hassan's performance was the main reason Suphanburi lost the match. In November 2015 Hassan signed a new two-year contract with the club.
In December 2016 after the relegation of Army United from Thai League T1, Hassan Sunny was released from the team, ended two years of his service at Thai club.
A few weeks after he was released, it was announced on Home United's Instagram and Facebook that they will sign the Singapore's number 1 keeper.
A graduate of the National Football Academy, Hassan forced his way into the Singapore against India after a superb debut season with Geylang United in 2003.
With midfielder Shahril Ishak, defender Baihakki Khaizan and winger Muhammad Ridhuan, he is part of the 'NFA Gang of Four', the quartet which has played together since their early teenage years and earned senior international honours in 2003.
He kept goal for the U-23 team that participated in the 2003 South-East Asian Games in Vietnam, before joining the Young Lions for the 2004 S.League season.
His first appearance in a Lions shirt came in August 2003 when the young keeper replaced Rezal Hassan at half-time in a friendly against the Japan Olympic team.
He had to wait until 18 February 2004 for his first cap for the Singapore though, which came against India in a World Cup qualifier.
He has been a regular fixture in the national team, though only as a substitute for Lionel Lewis . However, due to his poor performances in the Merdeka Cup in 2007 with the Under-23 team, he was dropped for the friendly against UAE and Jasper Chan was called up in his place.
His fine display at the 2007 SEA Games helped the Singapore Under-23 squad win the bronze medal - their first SEA Games medal since 1995.
He performed extremely well in a World Cup Qualifier match against Saudi Arabia, but Singapore lost the match 2-0.
On 28 July 2008, Hassan played the first half as part of a Singapore Selection side in a friendly match against the Brazil Olympics Team and made outstanding saves against Diego and Alexandre Pato's shots, letting in only three goals.
On November 30, 2014 AFF Suzuki Cup, Sunny has become a subject of controversy after failing to defend his goalpost from Malaysian counterattack. Malaysia which leading 1-2 finishes the match with open-goal scoring, making the final result of the match 1-3, Sunny has become a subject of parody in the aftermath of the game by Malaysian fans.
In 2016, UK based The Telegraph ranked him #18 on its list of the world's top 20 goalkeepers. He admitted to the Straits Times that he originally thought that it was a late April Fools joke.
He was selected as part of the Singapore Selection squad for The Sultan of Selangor’s Cup to be held on 6 May 2017.As of match played November 2016
. Caps and goals may not be correct.
Young Lions is ineligible for qualification to AFC competitions in their respective leagues.
SingaporeAFF Championship: 2004, 2007
Southeast Asian Games: Bronze Medal - 2007
Warriors FCS-League : 2014
S.League Player of the Year: 2014