That's a tough combination. Your best bet could be a 3-in-1 jacket, meaning a jacket that comes with both a shell and an insulated layer that zip together or apart depending on the weather.
Marmot's Bastione Component Jacket ($300) is a great example. It comes with a medium-length cut that won't look out of place on the street, and it's made with Marmot’s waterproof-breathable MemBrain fabric. The liner jacket is filled with light synthetic insulation and works as a standalone piece in mild conditions. And the shell's pit zips and removable hood mean you'll be in great shape if you take the Bastione into the mountains.
Another choice is Patagonia's Great Horn Parka ($379). It’s longer than the Bastione (the cut falls below the waist) and comes in conservative colors—black, dark green, and dark blue. It would perform well and look good around the town on cold and damp days, and with Patagonia’s reliable H2NO waterproofing treatment it can go into the woods, too. On super-cold days you'll want to add a layer of wool or fleece, but if it's warmer the liner serves as a nice little synthetic jacket. Like the Bastione, the Great Horn also comes with a removable hood.
One last option is REI's Salix Jacket ($199). As a single-piece jacket, it’s simpler than the Great Horn or Bastione, but the shell is made with REI’s proprietary Elements waterproof-breathable fabric and the insulation is top-flight PrimaLoft Eco. It’s light and packable enough to stuff into a pack for a winter hike or snowshoe trip, plus it's pit zips will help you regulate your temp. And if you go out to dinner in it, you won’t show up looking like a refugee from a polar expedition.