I've seen an opinion that patriarchal society was a logical consequence of settled agriculture as opposed to hunters-gatherers. Our traditional medieval society as we know it was not really fit for matriarchy.
But you have magic and mammoths, it may help! Let's achieving equality first and then see if we flip the bill.
Basically, it's a known argument in modern fantasy. Arguably it's because our modern readers would flip out at the real medieval amount of harassment and inequality the historically correct medieval society had. But for an in-world argument: magic!
Magic powers and prowess do not depend on gender. So, there are women fully capable to kick ass and unwilling to be treated as second-class citizen.
Let it boil for few centuries at least (magic is by far not new in your setting, I would guess), bam! Modern western society in the aspect of gender inequality.
Arguably, you'd need something that would give women a more advanced standing as men. As the society is medieval and ugly, they'd need to kick ass.
- More magic powers.
- Longer life.
- Biological affinity to magic.
- Mammoth taming and riding powers!
It depends on you and your story.
An original option is to let the society develop with magic from the basal gatherers and hunters. Both genders get magic powers. It is initially matriarchy, because someone need to keep fire burning and people fed. It never turns to patriarchy with the advent of agriculture. Women manage to defend their standing through magic or something, but the change to the mindset woman-belongs-to-a-man never happens. Ancient World and medieval history are very different now. Maybe, dunno, Aztec civilisation did not get that gory and that decadent.
I fear, you'd need to dive head-first into some anthropology books to make it believable. You would also need to redo the whole history since invention of agriculture.
There was a Russian fantasy/SciFi book (how can I not mention something from over there!) where one of societies was a classic matriarchy with a twist. Life was dangerous, women were capable of wielding some really cool (but also grim) life-threating magic, hence statistically lower life-spans and population bias. It was basically medieval Middle East / Japan with genders flipped: long-living, intriguing men behind political groups, reverse harems, that sort of thing. The situation was getting better in the book, but the traditional structure remained yet.