How Do You Get a Cat Adjusted to Living With a Dog?

It's World War 3 at my brother's house, and he could really use some advice!

My brother Connor officially became a homeowner late last year, but it hasn't really felt like home to him without a dog. Living at my mom's house, he was constantly surrounded by my mom's dog Pete and my dog Charles, and when you get used to coming home to their excited little faces, coming home to nothing makes the house seem kind of empty (trust me, I KNOW). He's been talking about getting a dog for a few weeks now, and this weekend he went out and adopted one! Mamba is about 1-years-old, and she's a boxer mix that used to be neglected. As soon as my brother saw her, he knew he had to have her. There's only one problem: Milo.

Milo is my brother's fiance's cat that lives in the new house with them. He is 7-years-old, and kind of keeps to himself. That's why he was NOT pleased when my brother brought Mamba home. There's been a lot of hissing and some jumpiness from Milo, which is very concerning to Jackie (Connor's fiance). Mamba has been socialized with cats and other dogs, so she is totally cool. Milo seems to be the one with the issue. From what I hear, this is a pretty typical problem to have. In the meantime, my brother crates Mamba at night and when they leave the house, so Milo doesn't feel threatened, and because my brother's fiance is worried about leaving the two alone together. There's been talk of bringing Mamba back to the shelter if things between the two species don't get better, but I know that would absolutely DEVASTATE my brother. Mamba already has him wrapped around her paw!

Have you ever had to deal with a situation like this where an old pet doesn't get along with a new pet? What did you do to make the transition easier? My brother could definitely use some advice! Feel free to leave some in the comments.