A product of the Scarlets academy, Priestland began his senior rugby career with Scarlets feeder club Carmarthen Quins, making six appearances in the 2005–06 season. In March 2006, he signed a professional development contract with the Scarlets. That summer, former Scarlets fly-half Stephen Jones returned to the region after two seasons in France with Clermont Auvergne, and for the 2006–07 season, Priestland served as the Scarlets’ third-choice fly-half behind Jones and Ceiron Thomas.

A member of the Wales under-19 squad that won the Grand Slam in the 2006 Six Nations, Priestland received his first senior call-up in January 2011 as part of a 28-man squad for the 2011 Six Nations Championship, as one of three fly-half options along with Stephen Jones and James Hook. After being an unused replacement for the 26–19 loss to England in the opening match, Priestland made his international debut on 12 February 2011, making a four-minute cameo at full-back in a 24–6 win over Scotland at Murrayfield. He was again named on the bench for Wales’ third match against Italy, but did not come on, before being left out of the squad entirely for the final two matches against Ireland and France.

Priestland’s good form for the Scarlets in the remainder of the 2010–11 season meant he returned to the Wales set-up for a match against the Barbarians in June 2011, coming on as a late substitute for Stephen Jones as Wales lost 31–28, before being included in a 39-man training squad ahead of the 2011 Rugby World Cup. In his first start for Wales, against England at Twickenham on 6 August 2011, Priestland was initially selected at full-back, but made a last-minute switch to fly-half after Stephen Jones was injured in the warm-up; Priestland kicked two conversions in a 23–19 loss. He again started at fly-half in the return match at the Millennium Stadium on 13 August, scoring two penalties before being replaced by Aled Brew at half-time; Wales won the match 19–9. Having seemingly done enough to earn a place in Warren Gatland’s World Cup squad, Priestland was left out of the final warm-up match against Argentina.

Priestland returned to fitness in time to play in Wales’ friendly match against Australia on 3 December 2011, starting ahead of Ospreys fly-half Dan Biggar. In addition to scoring two penalties, Priestland also crossed for his first international try as Wales lost 24–18. For the 2012 Six Nations Championship, Gatland selected Priestland and Hook as his two fly-half options, with Priestland starting all five matches, despite entering the tournament with lingering knee injuries. In the third match against England, he received the first yellow card of his international career. Full-back Leigh Halfpenny was given the goal-kicking duties during the tournament, so Priestland scored just three points: a penalty against Italy in the fourth match. Wales won all five of their matches in the 2012 Six Nations, claiming their third Grand Slam title in eight seasons.

However, following the Six Nations success, Wales suffered a 3–0 test series defeat to Australia on their summer tour and then failed to win any of their Autumn internationals; Priestland played in all seven of those matches, starting six of them and suffered a loss of confidence that saw him leave social media and start seeing a sports psychologist. In addition, he suffered a torn Achilles tendon in a match for the Scarlets in early December 2012, ruling him out of Wales’ Six Nations title defence in 2013. A recurrence of his Achilles injury meant Priestland also missed Wales’ summer tour of Japan, finally making his return to international rugby in November 2013, starting at fly-half in Wales’ opening match of the 2013 Autumn internationals against South Africa; he played the entire match, but Wales lost 24–15. Priestland then missed the next game against Argentina, before making substitute appearances against both Tonga and Australia.

Priestland regained the starting job for the 2014 Six Nations, but Wales won just two of the four matches in which he started, and he was dropped to the bench for the final game against Scotland. A knee injury suffered in the Scarlets’ final game of the 2013–14 season meant Priestland again missed Wales’ summer tour to South Africa in 2014. A groin strain for Biggar saw Priestland return to Wales action as a substitute in their 33–28 loss to Australia on 8 November, before starting the match against Fiji on 15 November; he kicked a conversion in each game, but both were in front of the posts after penalty tries. With Biggar now established as Wales’ first-choice fly-half, Priestland was named among the substitutes for every match in the 2015 Six Nations, coming on just twice, against France and Italy, and playing a total of just 16 minutes of rugby. As a result of his move to Bath at the end of the 2014–15 season, ahead of the 2015 Rugby World Cup, Priestland only appeared in one warm-up match, coming off the bench in a 16–10 away win over Ireland; he was also an unused substitute for a match against Italy a week later. Despite this lack of game time, Gatland named Priestland in his squad for the tournament, even giving him the starting role in the opening match against Uruguay. Priestland kicked seven of his eight conversion attempts, leading Wales to a 54–9 win. Biggar returned to start the remaining three pool matches, as well as the quarter-final loss to South Africa, but Priestland appeared off the bench each time.

In January 2015, it was announced that he would join English club Bath, on a two-year contract at the end of the 2014–15 season.

In September 2016, the Welsh Rugby Union (WRU) changed its selection policy, referred to as “Gatland’s Law”, so that only three players based outside Wales could be selected for international matches. With Priestland having to compete with the likes of George North, Jamie Roberts and his Bath teammate Taulupe Faletau for those places, he was left out of the Wales squad for the 2016 Autumn internationals. In September 2017, more than a year after his last cap, Priestland said he would be surprised if he played for Wales again. The WRU again changed its selection policy in October 2017, so that any player who signed a contract with a club outside Wales before then could be selected again, although their continued selection would depend on them returning to Wales at the end of their contracts; that meant Priestland was again eligible to play for Wales, and his club form led to him being picked for the 2017 Autumn internationals. He missed the first match against Australia, but then started the match against Georgia; he played the entire 80 minutes and was successful with all three of his kicks at goal, contributing eight of Wales’ points in a 13–6 win. He was on the bench for Wales’ third match against New Zealand, but replaced Biggar for the final 17 minutes of the game, earning his 50th test cap. Wales’ next match against South Africa was on 2 December, outside the World Rugby international window, which meant Priestland was unavailable as he had to return to his club side.

Priestland picked up a hamstring injury in January 2018, but nevertheless he was selected for the 2018 Six Nations; however, the injury proved severe enough that he did not take part in the tournament.