Great question! I went through this exact same scenario, with the exact same goal, and the outcome has been fantastic so far.
I started as the first design hire at a startup (I was the 14th hire). While I did both marketing and product design, my focus (and what we needed the most help with) was on product. As I leaned more and more into that, the marketing team hired a graphic designer to help them work on assets, etc., while I oversaw the product design side of things. But over time, she grew frustrated reporting to the CMO who knew little about how to help her grow as a designer/design leader. Meanwhile, she and I were constantly working together, and I became the de facto CD, mentoring her and overseeing her work. She and I were both eager to create a "design team" that was cross-functional, but we had a lot of resistance from the CMO and other founders.
How I eventually pitched it was:
- We'd function as an internal "design agency"
- I oversee the all aspects of product design, while also overseeing our creative vision and execution of the marketing design team
- This helps me ensure our customer experience from the moment they see an ad or hear about us, to signing up and logging in, is consistent and seamless
- The marketing designers would still have their work funneled to them directly from the marketing team, and their projects are tracked and managed by a marketing project manager
- This is all tracked in a shared project managing software (we use Asana). So I have a Product Design and Marketing Design team set up in Asana, and I can view both team projects. But I only directly manage the product design team's projects. The Marketing PM (and the marketing designer) manage the marketing design projects
The results have been fantastic. We have shared crits between product and marketing, and the swapping of ideas and perspective have led to numerous product and marketing design innovations here. The marketing designer, in particular, has felt a huge relief with having a whole design time backing her up and giving her a sense of place. And she's flourished, gaining so much more autonomy and freedom that she's ready to become a design leader herself. The net-net is that I've moved up to be the VP of Design, and I plan to promote her to being the CD. In time, I'll likely promote one of our product designers to become Director of Product design, so that I have two directors who oversee the product and marketing design, while I oversee both as a continued mentor, while dedicating more time to long-term vision, collaboration, and growth.
That outline of bullets I posted above were what I mapped out and proposed to the founders. I think I had to draw a diagram for them so they could see how it affected structure very little, but would have outsized impacts on efficiency and growth. That convinced them!
Hopefully you've already figured it out, but if not, this might work for you. Let me know how it goes!