Lexi Tobin was always "the planner" among her siblings and friends.

In 2019, Tobin helped her brother-in-law to-be plan his proposal. She ordered picture frames, strung lights across an archway, put candles in paper bags and set them up at her family's beach house in New York state. On the day, she shakily filmed her sister running down a white runner across the deck into her fiancé's arms.

A year later, she uploaded the footage to TikTok, on a whim. Within 30 minutes, more than 100,000 people watched the 41-second video, says Tobin, now 27 and living in New York City: "I think 9 million people watched it in the first 48 hours. I was completely blown away by the thrill of going viral."

A side hustle — planning other people's wedding proposals and posting bride-to-be's reactions on TikTok — was born. Tobin brought in six figures of revenue, including roughly $9,000 in median monthly revenue, during the 12-month period ending last month, according to documents reviewed by CNBC Make It.

In her highest-earning month, November 2023, Tobin's side hustle brought in more than $20,000.

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Tobin also plans other events: bridesmaids' proposals, brisses, engagement parties, birthday parties and, currently, her first wedding. Some of her revenue comes from her clients, and she keeps roughly 15% of her event planning fees as profit, she says. The rest of her revenue comes from brand and affiliate marketing deals through TikTok, which has nearly 330,000 followers, and Instagram.

She invests her side hustle earnings into the stock market, she says, living instead off the income she makes as a director of account management for an advertising tech company. Tobin spends up to 40 hours per week on her side hustle — a full-time job for most people — and roughly 60 hours per week at her day gig, where she earns "substantially more," she says.

Working up to 100 hours a week is taxing, but Tobin says she sees her side gig as a long-term project that's worth her time. And within the broader wedding industry, narrowing in on a specific niche like proposals is good for business, says Brian A.M. Green, an Atlanta-based upscale event planner who consults for WeddingPro, a vendor marketplace for wedding planning websites The Knot and WeddingWire.

One in four proposers now pay someone to help them pop the question, found The Knot's 2023 Real Wedding Study, which surveyed 10,000 newlywed couples. Right now, a proposal with five vendors could cost upwards of $10,000, a number that'll likely grow as the entire wedding industry gets more expensive, says Green.

Tobin typically hires one to three vendors per event, she notes. "It's all trial and error," she says. Her business took "a really long time to build up. I constantly have to remind my friends I've been doing this for five years, and it took time to get followers, to monetize content, to have an actual business."

In March, Tobin got married. Under twinkling lights, she walked down a candle-lined aisle in a embroidered gown with her parents on each arm. She took a deep breath as she reached her husband. Her beaming smile never wavered.

She posted it all to TikTok. More than 19 million people watched.

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