He was first discovered by Dutch coach Theo Bos, who went on to manage Schaars' U13 Vitesse youth team. Following successful development in Vitesse's youth team, he was promoted to the main squad for the final few games of the 2002–03 Eredivisie. He made his debut for the club on 9 March 2003 in a 4–1 away defeat to FC Utrecht. The following season he became a regular in Vitesse's side at the age of 19. In his first full season he played twenty one league games. In the 2004–05 Eredivisie, he helped his club reach a credible seventh place in the league contributing four goals in twenty-one appearances.
In the summer of 2005, he was signed by AZ Alkmaar. In his first full season at AZ, he suffered a serious hip injury at the beginning of the 2005–06 Eredivisie season which kept him out for several months. He returned in February 2006 against FC Utrecht in a 2–3 defeat. In the next season he missed most of it due to sustaining a number of serious injuries which also kept him out for the 2007–08 Eredivisie season.
His return came in the 2008–09 Eredivisie season, where he played a pivotal role in midfield along with David Mendes da Silva, Demy de Zeeuw and Maarten Martens to claim AZ's second league title in their history. Following the capture of the league title, he helped AZ claim their very first Johan Cruijff Shield in the 2009 edition of the tournament where AZ defeated SC Heerenveen. In the 2009–10 season he was named team captain of the side in which he held that position until the end of 2010–11 Eredivisie where he left the club in the summer after six seasons to join Sporting Lisbon of Portugal. During his time with AZ Alkmaar, the Dutch media frequently compared him to English midfielder Steven Gerrard due to his strong presence in midfield, his vision and his technical ability.
Schaars signed for the Portuguese side Sporting Clube de Portugal on 16 June 2011 on a transfer worth 850,000 euros. He was one of many new players bought by Luís Godinho Lopes under the management of Domingos Paciência who also bought other foreign players like Alberto Rodríguez, André Carrillo, Elias Trindade, Fabián Rinaudo, Oguchi Onyewu and Ricky van Wolfswinkel.
Under the management of Domingos Paciência and later on in the season Ricardo Sá Pinto he was a regular in Sporting's season. He scored his first goal for the club in a 3–2 away win against Rio Ave. The following matchday, Schaars scored his first goal at Estádio José de Alvalade. He opened the scoresheet in Sporting's 3–0 home win against Vitória de Setúbal. Following excellent displays during his first few months with Sporting, he attracted interest from several big European clubs of which included Barcelona and Olympique de Marseille. Despite Sporting having a disappointing domestic club finish where they finished behind FC Porto, Benfica and Braga in the 2011–12 Primeira Liga, Schaars played a pivotal role in Sporting's cup competitions. He helped Sporting reach the 2011–12 UEFA Europa League semi final stage where they lost to Athletic Bilbao, 4–3 on aggregate. Schaars was also part of the side that reached the 2012 Taça de Portugal Final where they lost to Académica de Coimbra, 1–0. Following the 2011–12 season, he was linked with a move to Premier League side Fulham.
Following the start of the 2012–13 Primeira Liga, he suffered an injury, for which he received surgery, which led him to miss several league, cup and European games. Schaars returned in Sporting's European away game to Videoton FC on 4 October 2012.
On 13 July 2013, it was announced that Schaars returned to the Eredivisie by making a transfer to PSV. PSV paid €1.6 million for both Schaars and his teammate Santiago Arias.
After his contract was not renewed at PSV, Schaars joined Heerenveen in July 2016.
His debut for the national team came on 16 August 2006, as he started in a friendly versus Republic of Ireland which the Netherlands won 4–0. Earlier that year he was the captain of the Dutch side that won the UEFA U-21 Championship 2006.
An ankle injury prevented Schaars from taking part in most of the 2006–07 season and he didn't play a single Eredivisie game during the following season. This effectively ruled out his chances of taking part in EURO 2008 with the Netherlands. Schaars was included in the preliminary squad for the 2010 FIFA World Cup in South Africa. On 27 May 2010, Netherlands manager Bert van Marwijk announced that the player would be part of the final squad of 23 participating in the competition.
Following frequent call ups to the Dutch side's qualification campaign to participate at UEFA Euro 2012, he was selected by manager by Van Marwijk to participate in the tournament finals. At Euro 2012, he did not participate in any of the group stage matches where his nation finished bottom of the group following three defeats to Denmark, Germany and Portugal.Statistics accurate as of last match played on 17 March 2017.
Statistics accurate as of last match played on 9 November 2016.
AZEredivisie Winner (1): 2008–09
Johan Cruijff Shield Winner (1): 2009
PSVEredivisie: Winner (2): 2014–15, 2015–16
Johan Cruijff Shield: Winner(1): 2015
Netherlands Under-21UEFA European Under-21 Football Championship Winner (1): 2006