The short answer is yes—most outdoor benches are easy to assemble, and you likely do not need a professional installer. If you can follow basic picture instructions, use an Allen key or a screwdriver, and set aside about 30 to 60 minutes, you can handle the typical flat-pack bench.
That said, "easy" depends on the bench. A simple two-seat wooden bench or a basic metal bench usually ships with a small hardware kit and a one-page manual. The process is mostly aligning pre-drilled holes, inserting bolts, adding washers, and tightening nuts. I always recommend having a second person only because some panels can be awkward to hold steady, not because the steps are difficult.
There are a few cases where calling a professional—or at least a handy friend—makes sense. If your bench is made from heavy stone, cast iron, or concrete, the weight alone can be a safety issue. Built-in electrical features such as LED lighting or heating elements also require proper wiring, so a licensed installer is worth it. Benches that must be anchored into concrete, brick, or pavers may need special drill bits and fasteners. Commercial-grade benches or modular systems with storage compartments can also be more complex than they look.
For most residential outdoor benches, though, DIY outdoor bench assembly is very realistic. Before you start, lay out all parts and hardware on a soft surface to avoid scratching the finish. Use a manual screwdriver or set your drill to low torque—over-tightening bolts is one of the most common mistakes and can crack wood or strip metal threads. If any bolts feel tight, back them out and check the alignment instead of forcing them.
In my experience, assembling an outdoor bench takes about 45 minutes from unboxing to final tightening. A professional installer may charge more than the bench itself for what is usually a simple job. Unless you have a heavy, wired, or permanently anchored model, grab the included tools, follow the instructions, and save the installation fee for cushions or a weatherproof cover.