Data from FE Trustnet and compiled by AJ Bell, showed Mr Hargreave's fund was one of 53 funds to have been managed by the same individual for the twenty years that Isas have existed.
Mr Hargreave, who launched his fund in 1998, has returned 2,470 per cent in these twenty years, compared with 550 per cent for the average fund in its sector, UK Smaller Companies.
The fund manager with the longest record of continuous service on a UK fund was Hugh Young, who has managed the Aberdeen Asia Pacific and Japan equity fund for 33 years, which returned 583 per cent over 20 years.
Among the largest investment trusts on the list was the £1.4bn City of London trust, run for the past 27 years by Job Curtis.
During that time the trust has increased its dividend every year, and the quarterly dividend now is greater than the annual dividend was when Mr Curtis took over.
Laura Suter, personal finance analyst at AJ Bell, said: "The average fund manager runs a fund for five years, so it’s pretty rare to find managers running the same fund for 20 years.
"The benefit of investing with a manager who has been running the same fund for two decades or more is that they will have invested through all market cycles, having experienced downturns as well as bull markets.
"These managers will also know their investment markets inside out, they will have been burnt by their past investment errors and will hopefully be wise to these traps in the future, and will know the companies in their sectors inside out."
She added: "The downside of this experience is that some investors might be put off investing with older managers for fear they’ll have lost their hunger for investing and become more cautious.
"Investors can check for signs of this by comparing the type of companies the fund manager is investing in now, compared to previous years.
"They should also be aware of contingency planning, as retirement is a real prospect with some managers who have been running portfolios for a long time."
The 20 largest funds with fund managers that have been in situ for at least 20 years:
20-year performance (%)
Peer group 20-year performance
Aberdeen Asia Pacific & Japan Equity
Aberdeen Asia Pacific Equity
Aberdeen New Dawn Investment Trust
£1.1bn in total
Aberforth Smaller Companies Trust plc
Alistair Whyte & Richard Newbery
City of London Investment Trust
The Mercantile Investment Trust
Herald Investment Trust plc
JPM Emerging Markets Investment Trust
JPM Emerging Markets
£2.3bn in total
Worldwide Healthcare Trust
Sven H Borho
Schroder Asia Pacific
Invesco Corporate Bond (UK)
Standard Life Investments UK Smaller Companies
Legg Mason IF Japan Equity
Jupiter Merlin Income Portfolio
Jupiter Worldwide Portfolio
Jupiter Growth Portfolio
John Chatfieild-Roberts and Algy Smith-Maxwell
£4.5bn in total
BlackRock GF World Mining
Lazard Emerging Markets
Liontrust UK Smaller Companies
Morgan Stanley US Growth
Dennis Lynch & Sam Chainani
Premier Multi-Asset Distribution
Marlborough Special Situations
Source: AJ Bell. Data accurate to 08/03/19.