Transit-accessible hiking areas near Los Angeles

get a transit app
for transit directions and arrival times.
Transit and Moovit are good.  Google maps and Apple maps do the job too

get a TAP card
for easy fare payments on most LA area transit systems  [TAP info]

get a map
a paper map and compass, or a GPS app that works without phone service, like these:
Gaia GPS - free offline map and track recording, ideal for dayhiking. optional subscription for more features
AllTrails - great trail guides and info. subscription required for offline use
Organic Maps - free offline route planning and track recording. open source, no account required
Hiking Project - free offline map and trail guides
Komoot - limited free offline map, additional maps available for purchase, no subscription

get knowledge
How to Ride Transit in Los Angeles: an excellent primer from Transiting Los Angeles
HikingGuy has soooo much great info for hikers
Hiking for Beginners:  essential hiking tips
What to Bring on a Day Hike:  water, snack, sun protection, etc
Poison Oak:  “leaves of three, let it be; if it’s hairy it’s a berry”
How dangerous are Bears?  Snakes?  Lions?  (not as bad as you thought)
Hiking 101:  the basics of mileage, elevation and routes
Hiking Glossary:  all the lingo

get more trails
HikingGuy
Modern Hiker
AllTrails
Hiking Project
Transit-accessible hikes near Los Angeles at AllTrails (best on desktop)

get a tent, maybe
and go camping at these transit-accessible campgrounds:
Crystal Cove  (see Laguna Beach - Crystal Cove)
Gould Mesa Trail Camp  (see Pasadena - Arroyo Seco)
Idlehour Trail Camp  (see Pasadena - Eaton Canyon)
Malibu Creek  (see Agoura Hills - Liberty Canyon)
Millard Trail Camp  (see Altadena - Millard Canyon)
Mt Lowe Trail Camp  (see Altadena - Echo Mountain)
Musch Trail Camp  (see Topanga - eastern trails)

get involved
Nature For All:  promoting transit to nature
Trail Angeles:  connecting forest volunteers
Mt Wilson Bicycling Association: maintaining the trails
Lowelifes Respectable Citizens Club: restoring and maintaining trails
Community Nature Connection:  increasing access to the outdoors for LA communities