Born 8 August 1992, Torquay.

Kieffer was on trial with Exeter City in December 2015 whilst at Viking in Norway. He also had a trial with Leyton Orient before signing for Forest Green Rovers.

Kieffer attended secondary school in Totnes and, with the demise of Torquay United's youth scheme, found himself playing for Paignton Saints in the South Devon League.  

His big break was to join Truro City in 2012 moving on to Dorchester Town early in 2013. After a brief - but successful - spell in Dorset, Kieffer signed for Yeovil Town in time for the 2013/14 season. After making fifty first-team appearances in the Championship and League One, Kieffer was released at the end of 2014/15 as the Somerset club faced up to a second relegation. 

It was at this point Kieffer played a summer and autumn in Norway, arriving at Exeter City once the season was over and he was back home in Devon.

After Norway, Kieffer's time at Forest Green Rovers was punctuated by a short loan at Torquay United, his hometown club. In one of his few games at Plainmoor, Kieffer scored a hat trick against Solihull Moors.

After Forest Green, Kieffer's career took an upward turn as he played for Ipswich Town, Barnsley, Wigan Athletic and Cardiff City.                

After being a particular success at Cardiff, Kieffer transferred to AFC Bournemouth for an undisclosed fee in January 2022. At Dean Court he quickly sustained an injury but returned in April for Bournemouth’s promotion run-in. He was to score the match-winner against Nottingham Forest which secured Bournemouth's promotion to the Premier League. With another goal the following week, Kieffer had scored four goals after just eighty-three minutes on the pitch since joining the Cherries.

Kieffer scored on his Premier League debut against Aston Villa in August 2022 and, after netting ten goals in 46 league and cup games, rejoined ipswich Town on loan at the start of February 2024.

Kieffer made his debut for Wales in 2019 and was to play a part in all three of Wales’ games in the 2022 World Cup in Qatar; starting two games and coming on as a substitute in the other. By the end of the tournament Kieffer had played 31 times for Wales scoring nine goals.