Fasth was born in Gothenburg, Sweden and in 1982, when he was 10 years old, his parents bought a summer house close to Lysegården Golf Club in Kungälv, north of Gothenburg. His family, with father Kristher, mother Inga-Lill and younger sister Jessica, used to spend their summers there and began playing golf. With a lot of friends also playing the game, young Fasth was always at the golf course, practised a lot and showed early talent.
At 16 years of age, he was adopted at the Swedish upper secondary sports school in Mark, outside Borås, to combine studying with golf training. The school won the Swedish School Championship in 1988, with Fasth winning individually, and represented Sweden at the international final in England.
He turned professional in 1993 and won three events on the second tier Challenge Tour that year. From 1994 until 2018, he played regularly on the European Tour, every season except for 1999.
He qualified for the U.S.-based PGA Tour at the 1997 Qualifying School, and played the PGA and European Tours concurrently in 1998, without success and in 1999 found himself back on the Challenge Tour. Since then he concentrated on playing in Europe.
He finished tenth on the 2001 European Tour Order of Merit, after being lone runner-up to David Duval at the 2001 Open Championship at Royal Lytham & St Annes Golf Club, England. After a closing 67, Fasth advanced from tied 20th and was the leader in the clubhouse for two hours, before late starting Duval also closed with 67 and won by three strokes.
His 2001 performances enabled Fasth to qualify for the European team at the 2002 Ryder Cup, at The Belfry, England, where he contributed to a European 15½ to 12½ victory, with a halved match against Paul Azinger in the Sunday singles.
His appearance at the 2005 New Zealand Open was capped as Fasth kept a cool head to put down Miles Tunnicliff, sinking a birdie putt at the second playoff hole after the pair had finished a stunning 22-under par.
At the first playoff hole, the 18th, Tunnicliff’s approach shot flew the green and bounced off 12-year-old June Eun Jung’s head, ricocheting 30m into light green-side rough. She was taken to the hospital. Showing steely nerves, he plopped a gentle pitch close enough to hole the putt.
In both 2005 and 2006, Fasth won twice on the European Tour including his 2005 New Zealand Open crown and finished 13th and 15th respectively on the Order of Merit.
He finished lone fourth at the 2007 U.S. Open at Oakmont Country Club, Pennsylvania, two strokes after winner Ángel Cabrera. The week after, Fasth won his sixth European Tour event, the BMW International Open in Munich, Germany, ahead of home hero Bernhard Langer. These performances helped Fasth to a career-best fifth-place finish on the 2007 European Tour Order of Merit.
Fasth has featured in the top 20 of the Official World Golf Ranking with a personal best of 18th for three weeks in October 2007.