A defender from Welwyn Garden City, Harry Toffolo signed for Norwich City as an Under-14 in 2008. He is a graduate of the Hatfield development centre and was previously with Stevenage Borough. He is described as a solid left-sided defender who gets himself into advanced attacking positions to support his team.

.On 31st March 2011, it was announced that he and eight others would join Norwich's Academy in the summer of 2011. Academy manager Ricky Martin said: “I would like to wish all nine boys the best of luck and hope they enjoy long and successful careers at Norwich City.”

At the end of February 2013, Toffolo was called up to the England Under-18 squad for the clash against Belgium in March 2013.

Toffolo played in both FA Youth Cup semi-final matches against Notts Forest which the Canaries won to progess to the FA Youth Cup final. He also played in both legs of the final which the Canaries won 1-0 and 3-2 against Chelsea to secure victory in the FA Youth Cup for the first time in 30 years.

He was on the bench for Norwich in the Capital One Cup win over Crawley on 26th August 2014

On 20th October 2014, he was loaned to Swindon Town for a month to gain first team experience. This was extended to the end of the season due to his good form for Swindon's first team. At the end of March 2015, he was called up into the England Under-20 squad and made his debut in a win against the USA.

Toffolo played in Swindon's League One play off final defeat to Preston.

After a promising pre-season in which Harry got plenty of match action with inujuries to Martin Olsson and the delay in signing Robbie Brady, he made his first team debut on 25th August 2015 in the 2-1 Capital One Cup win at Rotherham. He was called up in October 2015 for the England Under-20s again and played the full 90 minutes in a 3-0 win over Holland on 7th October 2015 as part of the Mercades Elite Cup.

On 19th October 2015, he was loaned to Rotherham on a one month loan deal. This was extended until January 2016 but he made just seven appearances for the Millers. After returning to Norwich, he was loaned to Peterborough on 21st January 2016 until the end of the season.

Toffolo made his England Under-20 debut in the 2-1 defeat to Canada in late March 2016.

Toffolo was surprisingly loaned to Scunthorpe on 31st August 2016 until January 2017 and made his Iron debut in a 4-0 win over Southend on 10th September. This was extended in January 2017 until the end of the 2016/17 season.

Norwich activated a one year extension to Toffolo's contract in May 2017.

On 31st August 2017, Toffolo was loaned out again joining Doncaster Rovers until 3rd January 2018. This was somewhat of a surprise to Norwich fans as they had not been impressed with the form of new signing James Husband whilst Jamal Lewis, the club's only other left-back, was out injured until the New Year (Marco Stiepermann can play on the left if Norwich use a three man central defence). A broken foot ended his loan prematurely in mid-December with Toffolo having played 17 games for Rovers.

On 29th January 2018, Toffolo joined Millwall on a free transfer. He was released on 18th May 2018 without making a single first team appearance. He then signed a two year contract with Lincoln City on 12th June 2018.

On 13th April 2019, Lincoln secured promotion to League One and claimed the League Title on 22nd April 2019 drawing 0-0 with Tranmere. He was named in the Professional Footballer's Association League Two team of the year along with teammates Jason Shackell, Neal Eardley and John Akinde.

On 17th January 2020 Toffolo joined Huddersfield,managed by former Lincoln manager Danny Crowley, for an undisclosed fee. He signed a two and a half year deal.

Toffolo was in the Huddersfield team that lost the Championship play-off final to Notts Forest on 29th May 2022. He was then linked with a switch to Forest on 3rd July 2022 witha transfer fee of £2.5million being mentioned. He had made 46 appearances for Huddersfield in 2021/22. Toffolo and his Huddersfield teammate Lewis O'Brien completed a £10million move to Forest on 20th July 2022. He said "I'm extremely delighted to be here. To have the opportunity to play in the Premier League is something I couldn't turn down.I'm excited, really looking forward to meeting the lads and pushing forward in this division. We've got a fantastic squad being put together.

In July 2023 Toffolo was charged by the Football Association for breaching its betting rules 375 times between 22nd January 2014 and 18th March 2017. On 13th September 2023 the FA banned him for five months but this was suspended until the end of 2024/25. He was fined £20,956.