When ISAs were first introduced in 1999, very few people imagined they would eventually create thousands of millionaires.

Yet today, according to HMRC data, there are now more than 5,000 ISA millionaires in the UK, with some investors holding portfolios worth several million pounds entirely sheltered from tax. The number has also risen dramatically over the last decade as markets, contributions and compounding have all quietly done the heavy lifting.

One of the reasons ISAs are so powerful is that they allow your investments to grow free from capital gains tax and dividend tax. As a result, more of your money remains invested and continues compounding over time.

Many people assume becoming an ISA millionaire requires exceptional investment skill. In reality, time is often the most important ingredient.

For example, someone investing the full £20,000 ISA allowance each year and achieving an average annual return of 5% could potentially build a £1 million portfolio in around 26 years.

The most interesting part is what happens towards the end of the journey. Even though the annual contributions remain the same, the portfolio begins growing much faster as compounding takes over.

This is why many successful investors focus less on trying to get rich quickly and more on simply staying invested for as long as possible.

One of the biggest misconceptions in investing is that successful investors consistently outperform the market through superior intelligence or an ability to predict the future. In reality, long-term investing success is often driven more by behaviour than brilliance.

Many ISA millionaires likely followed a surprisingly simple approach. They invested regularly, stayed invested during market downturns, avoided panic selling and largely ignored financial noise. They also kept costs low, allowing more of their returns to remain invested and compound over time.

Human beings are naturally drawn to excitement, predictions and quick wins. Yet the rise of the ISA millionaire suggests a different reality. Some of the most successful investors are not those making the most decisions, but those making the fewest. More often than not, they simply developed a sensible plan and stuck with it for decades.

Millions of people across the UK buy lottery tickets every week in the hope of becoming wealthy. Far fewer consistently invest into an ISA.

Yet many ISA millionaires have achieved exactly that—not through luck, but through patience, discipline and the power of compounding. They accepted that building wealth would take years, that markets would sometimes fall and that progress would often feel slow.

The lesson is not that everyone will become an ISA millionaire. It is that long-term investing works far more often than many people realise. While headlines celebrate overnight success, most wealth is built quietly through regular investing and allowing compounding to do its work.

The rise of the ISA millionaire is a reminder that successful investing is rarely about brilliance or perfect timing. More often, it is about consistency, patience and staying invested long enough for compounding to work its magic.

The number of ISA millionaires is likely to continue growing over the coming decades. Not because thousands of people suddenly become investing experts, but because compounding is incredibly powerful when combined with time, discipline and the tax advantages that ISAs provide.

If you're not currently investing through an ISA, ask yourself a simple question: should I be?

For many investors, the journey to financial independence starts not with a brilliant investment idea, but with making full use of the opportunities already available to them.

If there's a topic you'd like me to cover in a future edition of The Compounder, leave a comment below. Chances are, if you're wondering about it, somebody else is too.

The Compounder
Long-term investing made simple.