These parks are all linked together by both a paved multi-use trail and by unpaved hiking trails. This is a very popular park, especially on weekends.

The PATH trail connects Mason Mill Park to Medlock Park. The PATH trail is partly paved and partly boardwalk. It is intended for high volumes of pedestrians and cyclists. A map is available online.
The hills along the PATH route are minimal. The PATH trail has several nice views of South Fork Peachtree Creek from an elevated boardwalk.

On the other hand, the off-road trails have many very steep hills with some tricky footing, often with exposed roots. There are some blazes on the off-road trails: red, blue, and white rectangles painted on trees.

[Edit, November 2019:] The PATH has recently been extended from Mason Mill Park north all the way to N Druid Hills Rd and also west from Melton Park all the way to Emory University. It’s now quite convenient to combine a trip to these parks with Lullwater Preserve.

Finding the primary trailhead: Make a right from N Druid Hills Rd onto Clairmont Rd. Take the second left onto McConnell Dr. A large, free parking lot is available at Mason Mill Park.

Address (approximate): 1340 McConnell Dr., Decatur, GA 30033
Cost of admission: None
Governance: DeKalb County
Acres: 140+
Approximate miles of trails: 4
Hours: PATH: dawn to dusk; parks: 7:00am to sunset
Miles from downtown Atlanta: 5
MARTA directions: Take the 19 bus from Chamblee or Decatur station to Clairmont Rd@McConnell Dr. From there, walk East on McConnell Dr. The PATH trails begin at the end of McConnell Dr., heading north and east.
Cycling directions: My preferred route is to take MARTA or the PATH to Decatur Station. From there, go east on Sycamore St. then left on Sycamore Pl. Take the first right on E Ponce and then the first left on Glendale. Take a slight left to stay on Glendale. At the end of Glendale, take a left on Forkner. Cross Church St. and continue straight(ish) on Medlock. Take Medlock all the way to the end, then turn left on Gaylemont Cir. Once you see Medlock Park, turn left into it.
Parking: Parking is available at Medlock and Mason Mill Parks.
Trail surface: The PATH trail is paved and boardwalk. Other trails are natural surface.
Geocaching: Yes
Guidebook mentions: Chapter 4 in Hiking Atlanta's Hidden Forests

Features:

Mason Mill Park
- Bike racks
- Playground
- Multi-use field
- Recreation center
- Tennis courts
- Picnic area
- Garden
- Bathroom
- Dog park (separate from the main park)
Medlock Park
- Baseball and softball fields
- Playground
- Swimming pool
- Picnic area