The trained teacher commenced his career as player with SC Zug, FC Baar, FC Emmenbrücke, and FC Wettingen. After this followed assignments as professional with Grasshopper Club Zürich and FC Basel. In his time with Grasshoppers he won the Swiss Championship and Cup.
His managerial career began with FC Emmenbrücke where he was player manager from 1995 to 1997. After this he spent a season with each, FC Winterthur and FC Baden.
In March 1999 he took on German second division side SSV Ulm 1846, then on 5th place and in the course of the season's last 11 matchdays led them to the third place, and thus the first ever promotion to the Bundesliga. After 24 matches there the club was closer to an UEFA Cup rank then to the relegation zone, but a crisis thereafter saw the SSV 1846 passed down again after just a single season. After bad results at the beginning of the new year in the second division Andermatt was let go already in September. This maneuver did not help the club at all and by the end of that season Ulm had gone all the way to the third division.
From June 2001 until his premature exit in March 2002 he was hired by German side Eintracht Frankfurt which was just relegated to the second division in the hope that he might lead the team to an immediate return to the Bundesliga. After the fulfillment of this aspiration became more and more unrealistic he was replaced halfway through the second half of the season.
From 2003 to 2005 he coached FC Vaduz in Liechtenstein and alongside this the national side of the principality, where he could achieve some surprising results when drawing. 2-2 against Portugal, the losing finalists of the Euro 2004. Before this result, Liechtenstein had lost all of its previous 20 World Cup qualifiers. The team also caused a shock in the return match at the end of the group phase when Benjamin Fischer scored and Liechtenstein led at half time before eventually losing 2-1. Liechtenstein under andermatt also won 4-0 at Luxembourg. On the other hand, Liechtenstein are the only country ever to lose to San Marino with a 0-1 loss in a friendly match on 28 April 2004.
The club side led in the Swiss second division for a long stretch of the 2004/05 season. At the end of the season FC Vaduz had a minor crisis which saw the slipping and Andermatt was fired four rounds before the end of the season. Vaduz missed out on promotion in the play-off matches versus Neuchâtel Xamax.
Andermatt however retained his position with the national team until October 2006 when he took the manager job with BSC Young Boys in Berne. There he finished his first season 4th.