Sandomierski's career began at Jagiellonia Bialystok, starting in the reserve side from 2006. He played in four games and then was loaned out to Lech Poznan. He made two appearances in the third division, after which, he returned to Jagiellonia. In the 2007–08 season, due to an injury to Jacek Banaszyński, Sandomierski made his début in a first league match with Cracovia in which he conceded two goals. By the end of the competition, he appeared in four other matches. Sandomierski then sustained a serious injury which excluded him from the game for half a year. In January 2009 he was loaned to Ruch Wysokie Mazowieckie, where he played as first choice goalkeeper. Prior to the 2009/2010 season, he returned to Bialystok. Sandomierski got his chance for Jagiellonia after Rafał Gikiewicz suffered a poor run of form. During his time at Jagiellonia, for six matches in a row, Sandomierski kept a clean sheet. His streak lasted 564 minutes, a club record, and ended in a clash with Ruch Chorzów.
On 31 August 2012, Sandomierski signed on a season long loan with Blackburn Rovers with a view to signing longterm.
He made his debut for Rovers against Cardiff City in a 3–0 defeat conceding 2 goals one of which was a penalty, coming on for injured counterpart Jake Kean in the 52nd minute. Sandomierski's second appearance came against Sheffield Wednesday after it was announced that Kean would miss the remainder of the season through injury. Caretaker manager Gary Bowyer expressed his faith in the Polish international, however Rovers were beaten 3–2 on the day. His next match saw him gain two impressive clean sheets against Derby County and Huddersfield Town. He then conceded 4 goals against Watford in a not so impressive match. In his next match against Millwall he only conceded 1 goal but had a fairly comfortable game.