The company, which is holding its AGM today, said in a trading update that total customer numbers had increased by 4% in the quarter to 241,152.
Assets under administration grew 5% to £54.7bn.
AJ Bell’s growth in the quarter mainly came from its platform business with platform customer numbers up by 9,033 to 227,202.
Its platform AUA increased by 5% over the quarter to £47.2bn. The Advised Platform AUA was up 4% to £35.2 billion while the D2C Platform AUA was up 8% to £12bn.
Underlying platform inflows increased to £1.3 billion (2018: £1.2 billion).
Andy Bell, chief executive at AJ Bell, said: “Our investment platform continues to attract new customers and assets as we maintain our focus on being the easiest platform to use in order to help our customers to invest.
“December saw the unlocking of the parliamentary deadlock, which boosted markets towards the end of the period. Combined with positive net inflows onto the platform, this helped our platform assets under administration increase to a new high of £47.2 billion, up 27% compared to a year ago.
“Whilst Brexit is by no means resolved, a majority Government does at least provide some clearer direction which many investors will appreciate as they consider making the most of their ISA and pension allowances before the end of the tax year.”
• Rival platform Transact also reported strong trading this week. Parent company IntegraFin said quarterly Funds Under Direction totalled £39.3bn, up 4% over the quarter. Chief executive Ian Taylor said the company had seen its " highest ever first quarter inflows" and Funds Under Direction were 24% higher in December than a year earlier.