Many investors might associate millionaire-making stocks with New York rather than London. The likes of Nvidia, Tesla and Monster Beverage may spring to mind. Yet some FTSE stocks have created riches too.

Indeed, a £20k investment in either of these two UK shares would have grown into a small fortune as they transitioned from small-cap stocks to blue-chip companies in the FTSE 100.

But are they still worth buying today?

JD Sports

First up, we have JD Sports Fashion (LSE: JD). Shares of the sportswear retailer have moved from a split-adjusted 2p in June 2004 to 126p today.

That’s a mammoth 5,909% gain that would have turned £20k into around £1.2m!

On top of this, our hypothetical investor would also have bagged a load of dividends.

So what has gone so right at the company over the past two decades?

Well, unlike many high street retailers, JD embraced e-commerce early on. It developed a seamless omnichannel experience, allowing customers to shop online, pick up in-store, or return online purchases in person.

And it developed strong partnerships with major brands like Nike to sell limited-edition releases and promote trends. This helped it carve out a strong brand image with a younger, fashion-conscious demographic.

Another significant factor in the firm’s success has been global growth, both organically and through acquisitions. Revenue rocketed from £471m in 2004 to £10.5bn in the latest financial year. Profits have soared alongside this.

Today though, the company has hit a growth speed bump, with economic weakness to blame. This has sent the share price down 23% year to date.

Weak consumer spending remains the main risk here, I’d say. We don’t know if and when growth will be kickstarted again.

Having said that, the stock now looks cheap, trading on a forward price-to-earnings (P/E) ratio of 10. I think it might be worth considering and it remains on my watchlist.

Ashtead

Next, we have Ashtead Group (LSE: AHT), whose long-term returns have been truly epic. Shares of the equipment rental company have gone from a mere 22p in early June 2004 to 5,560p today.

That’s a mind-boggling gain of 25,172%! In other words, a £20k investment back then would now be worth around £5m. There would have been a rising dividend too, boosting the total return.

Both of these examples really demonstrate how powerful buy-and-hold investing can be.

Similar to JD Sports, the company has grown tremendously through international expansion and numerous acquisitions.

Indeed, Ashtead is now the second-largest plant hire firm in North America, up from the fourth-largest in 2004. And it continues to hoover up smaller competitors to build market share.

However, with the vast majority of its operations now across the pond, it’s exposed to any cyclical slowdowns in construction there. That’s always a risk with Ashtead.

Nevertheless, I would still buy the stock today if I wasn’t already a shareholder. That’s because the company is poised to benefit from the construction boom in the US, where massive government spending has been approved to upgrade infrastructure and to onshore manufacturing currently outsourced to Asia.

The stock’s millionaire-minting days may be behind it, but I reckon it will still outperform the FTSE 100 long term. It’s one of my favourite Footsie shares.