Sommer made his debut for the Switzerland national team in He moved to the Basel youth setup in He signed his first professional contract with Basel in He became the first-choice goalkeeper in the under squad almost immediately, fending off contenders like Jayson Leutwiler and Orkan Avci , and was third-choice for the senior squad behind Franco Costanzo and Louis Crayton.
During the summer of , Sommer signed a new contract lasting until , and was loaned out to Liechtensteiner club Vaduz to gain first-team experience in an environment somewhat more competitive than the reserves in the Swiss 1. He was made the first-choice goalkeeper and played 33 matches over the course of the β08 Swiss Challenge League season, playing a major part in Vaduz's promotion to the Swiss Super League.
His loan deal was then extended to last until January Sommer was recalled to Basel on 7 January because the club's first-choice goalkeeper, Franco Costanzo, was injured. He played six matches for Basel up until the end of the season. On 16 June, he joined Grasshoppers on a one-season-long loan. On 14 June , Sommer signed a five-year contract with Basel. In March , Basel announced it would not offer Costanzo a contract extension, therefore making Sommer the club's first-choice goalkeeper.
Sommer played five Super League matches during the β11 season and so was part of the championship-winning team. At the end of the β13 season , Sommer won the league title for the third consecutive time, [ 8 ] having played in all 36 league matches.
Basel also finished as runners-up in the Swiss Cup. Basel was eliminated 2β5 on aggregate. At the end of the β14 season , Sommer won the league championship with Basel. In the β14 Champions League , Basel finished the group stage in third position to qualify for Europa League knockout phase , where they advanced as far as the quarter-finals. Sommer totaled 58 appearances: 35 League, two Cup, six Champions League and six Europa League, as well five in the test games.
