Katie, who starred on Season 10 of Married at First Sight on Lifetime, made the big announcement of her engagement on Thursday, which was Christmas Eve, although she did not specify which day Brandon actually popped the question.
Shortly after her first post, Katie posted two images of Brandon and herself walking hand-in-hand.
"If you told me at the beginning of 2020 that this was how it would end, I'd never believe you," Katie shared with her followers on Instagram.
"Although this year has definitely not been easy for anyone, for me it's been the best one yet. This year brought me my best friend, my person, the love of my life, and the person I'll continue to choose forever."
Katie continued, "Life is so unpredictable and so amazing. I can't wait to see what 2021 has in store, but mostly I can't wait to be your wife."
Brandon is an Air Force officer from Chattanooga, TN, who's currently attending Marine Corps University and living in the Washington, D.C. area.
FOLLOW REALITY TV WORLD ON THE ALL-NEW GOOGLE NEWS!
Reality TV World is now available on the all-new Google News app and website. Click here to visit our Google News page, and then click FOLLOW to add us as a news source!
"Spent the weekend celebrating this amazing guy's mama in his hometown and couldn't be happier #happymothersday," Katie captioned a photo of the cute couple standing outside on a sidewalk.
Katie had mentioned Brandon and their relationship beforehand -- but only in her Instagram Stories and Instagram Live postings.
In early May, Katie dished the details of how she and Brandon first met during an hour-long Instagram Live session.
"Brand new boyfriend! Me and Brandon met on Hinge actually, which is a dating app," Katie gushed.
"I was getting really frustrated with dating apps, I'll be honest. I was over it, but after this whole marriage thing happened, I kind of had a different outlook on it."
"I think I felt like I was getting older and I was ready to find something that I wasn't finding," she continued. "But I think this whole experience reminded me of how young I actually am and that I do have time to find that."
Katie cheated on Derek with her ex-boyfriend two weeks after "Decision Day" filmed last year and then she and Derek called it quits on their marriage on the one-month anniversary of "Decision Day" when they had taken a vacation to Nashville, TN.
Katie complained Derek was "fake" and her marriage lacked physical intimacy because Derek allegedly never initiated in the bedroom, but Derek vented about how he couldn't trust Katie.
"I got back on [dating apps]... [and Brandon] asked me out to dinner right before all this quarantine stuff happened, and he ended up picking my favorite restaurant without even knowing it's my favorite restaurant," Katie shared on her Instagram Live.
The couple's first date presumably took place in mid-March given she and Brandon went out before quarantine regulations were put into place across different states in the U.S. amid the ongoing coronavirus pandemic.
"We ended up talking just about the things we want in a relationship for five hours after we had dinner," Katie said.
"So we got to know each other really well, just on the first date. And then all the quarantine stuff happened and we live in the same neighborhood, so we've spent a lot of time together and things are going really well! So, we'll see!"
Prior to "Decision Day," Derek and Katie weathered a lot of ups and downs.
Katie spent a lot of the Married at First Sight season complaining about Derek's boy-like personality and "unrealistic" dreams. She admittedly started fights with Derek to get attention from him and test his feelings for her.
Although Derek also made some mistakes in their relationship, he was shown being attentive to Katie and caring about her needs.
Derek never seemed to give up on the potential they had until Katie confessed to sleeping with her ex after "Decision Day" and dumped Derek.
Katie admitted to having a physical "slip-up" with her ex after "Decision Day," but she completely denied ever having an affair once she and Derek returned to Washington, D.C. from their honeymoon in Panama.
Derek still appears to be single, according to his Instagram account, although he told Married at First Sight alums Jamie Otis and Doug Hehner during a May episode of the couple's Hot Marriage Cool Parents podcast he's "open to finding love."
Also during the podcast, Derek revealed what he thought Katie's "biggest issue" was in their marriage and when he believed their relationship started going downhill.