Williamson attended Tauranga Boys' College from 2004–2008 where he was Head Boy in his final year and was coached by Josh Simms.
After a difficult winter tour with the Emerging Players' Squad, Williamson bounced back in fine style for Northern Districts averaging almost a hundred in the domestic one-day tournament, culminating in scoring 69 off 71 balls the final victory over Auckland Aces.
On 24 March 2010 Williamson was named in the New Zealand Test squad for the second Test against Australia, but ultimately he did not play in the match.
Williamson made his One-Day International debut against India on 10 August 2010. He was dismissed for a 9th ball duck. In his second match, he was bowled by Angelo Mathews for a second ball duck. He scored his maiden ODI century against Bangladesh on 14 October 2010 in Dhaka and hence became the youngest centurion in New Zealand cricket history. Due to his performance on the Bangladesh tour where New Zealand suffered a 4–0 whitewash, Williamson was selected in the New Zealand Test squad for the tour of India that followed.
Williamson scored his highest test match score of 161 against West Indies in June 2014.It was his second test century of the series and helped secure a rare away test series victory for his side.Williamson finished as the leading overall run scorer in the series with 413 runs, and was only denied a double century by rain which forced skipper Brendon McCallum to declare in the interest of a result in the match. 
Williamson is currently third on the list (behind Corey Anderson and Ross Taylor) for the highest ever strike-rated century by a New Zealander in a One-Day International. He achieved during his 100* off 69 balls against Zimbabwe at Bulawayo, but a fine innings of 99* by Malcolm Waller ensured that he ended up on the losing side.