Investing in 2020 was not for the faint of heart. The broader market set a number of records last year, including the swiftest bear market decline in history, as well as the most ferocious bounce from a bear market bottom to new highs.

As we look ahead to 2021, it may well be time for mid-cap stocks -- which range from a market of $2 billion to $10 billion -- to shine. Generally, mid-cap stocks have more established operating models than small-cap stocks, but they can offer more long-term upside potential than mature large-cap stocks. If getting richer in 2021 is one of your goals, the following five mid-cap stocks can help you accomplish that.

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Cresco Labs

Other than electric vehicles, there's not a hotter industry right now than marijuana. Even if it were to pull back a bit after a red-hot start to 2021, U.S. multistate operator (MSO) Cresco Labs (OTC:CRLBF) would make for a smart purchase.

Like most MSOs, Cresco has retail operations, though its retail footprint is a bit smaller than other billion-dollar cannabis companies. It currently holds 29 dispensary licenses and has opened 20 stores. However, if its proposed all-stock acquisition of Florida's Bluma Wellness closes, it will hold 44 licenses and have 27 open locations. It has the maximum allowable retail locations (10) open in Illinois, which is a limited license state that's fully capable of more than $1 billion in sales by 2024. 

But what really allows Cresco to stand out is its top-tier wholesale operation. The acquisition of Origin House in January 2020 was big because it gave Cresco a cannabis distribution license in California, the largest pot market in the world. Cresco is able to place pot products into more than 575 dispensaries in the Golden State. This figure should continue to rise as new retail locations are opened.

Cresco could move to recurring profitability in 2021.

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SSR Mining

Taking my inherent bias into account (it's my largest holding), it should be a banner year for gold stock SSR Mining (NASDAQ:SSRM).

The outlook for physical gold remains exceptionally lustrous. The Federal Reserve has no intention of considering a rate hike before 2024, and a record amount of investment-grade debt was recently sporting a negative yield. With low yields predominating in developed markets worldwide, gold will increasingly be turned to as a store of value.

SSR Mining isn't just going to benefit from a higher gold price, though that certainly helps. The company completed a merger of equals with Turkey's Alacer Gold last year. Now reporting as a combined company, the new SSR shouldn't have trouble topping 700,000 ounces of gold equivalent production, with the Seabee, Marigold, and Copler mines all capable of record output this year.

Historically, I've found gold stocks to be fairly valued at 10 times their operating cash flow. SSR Mining is currently valued at a multiple of 5 times Wall Street's projected operating cash flow in 2021. That's a bargain.

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Ping Identity

Though there are many industries that offer sustainable double-digit growth potential this decade, cybersecurity might be the safest bet for success. Businesses of all sizes need to protect their data and that of their customers no matter how the economy is doing. This is where Ping Identity (NYSE:PING) comes into play.

Ping Identity is responsible for creating identity verification solutions that lean on artificial intelligence and machine learning to grow smarter over time. The more events Ping's security solutions oversee, the better they are at identifying potential threats and keeping clients' data safe.

Despite being hit a bit harder than its peers during the coronavirus recession, Ping Identity has delivered a number of positives. Net cash provided by its operating activities more than doubled during the first nine months of 2020 ($20 million vs. $8.5 million in 2019). The company's annual recurring revenue jumped 17% by the end of September to $242.6 million. 

Since Ping generates well over 90% of its revenue from high-margin subscriptions, it should be able to build on its profitability in 2021.

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White Mountains Insurance

If you love truly off-the-radar mid-cap stocks, White Mountains Insurance Group (NYSE:WTM) might be the stock for you.

White Mountains is sort of like a mini-Berkshire Hathaway, just much more conservative. It has five major investments, one of which recently went public: MediaAlpha. True to its namesake, it holds positions in companies that handle municipal bond insurance (HG Global) and reinsurance (BAM). It also owns stakes in companies that provide wealth management services (Kudu) and marketing technology (MediaAlpha). Its selection of low-risk, fee-based businesses has slowly but surely pushed the company's book value higher.

White Mountains Insurance also invests in fixed-income assets and equities (most often passive exchange-traded funds). Though fixed-income assets have a lower average annual return than investing in individual stocks or stock-focused exchange-traded funds, fixed-income assets are also considerably less volatile and provide predictable returns.

White Mountains ended an 11-year total return win streak in 2020. The company is a good bet to start a new winning streak in 2021.

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Exelixis

Last but not least, cancer-drug developer Exelixis (NASDAQ:EXEL) has what it takes to make investors richer in 2021.

The undisputed growth driver for Exelixis continues to be Cabometyx, which is approved as a treatment for first- and second-line renal cell carcinoma (RCC) and advanced hepatocellular carcinoma (HCC). These indications should help Cabometyx surpass $1 billion in annual sales this year.

Exelixis is testing Cabometyx as a monotherapy or combination therapy in more than 70 ongoing clinical trials. One of these studies was in combination with Bristol Myers Squibb's immunotherapy blockbuster Opdivo, and it led to a positive result in phase 3 trials for first-line RCC. Cabometyx's role in cancer treatment appears likely to grow over time.

Meanwhile, Exelixis has reignited its internal research engine to expand beyond Cabometyx, and the company's cash pile seems to grow with each quarter. It's an absolute bargain among biotech stocks.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.