Until 1912, Key Largo and the rest of the Florida Key islands were largely inaccessible. With the creation of the Florida East Coast Railway, the islands opened up and tourism began to flourish.  

Public transportation on Key Largo is sketchy.  Connected by the modernized Overseas Highway, which was built in 1938 when a hurricane destroyed the original railroad system, Key Largo is easily accessible by vehicle, be it car, bus, or taxi.  Most visitors will rely on a rental car to get around.

Key Largo is about an hour's drive from southern Florida's Miami International airport and about 2 hours from Key West, with its airport Within Key Largo itself, a number of taxi services also provide limousine service for getting around town.     

Boat travel is a nice option if you're interested in a trip around the island by water, or for checking out some of Key Largo's sister islands.

 

The trip to and from the Florida Keys and any one of South Florida airports doesn't have to be expensive or boring. The Monroe County and Miami-Dade public transportation systems are clean, safe, reliable and timely.
 
The truly Green way to travel is to use public transportation. It is unfortunate that the Dive Capitol of the world " Key Largo" and the Florida Keys do not promote this form of travel. The promoted ways to the Keys is a van shuttle, highway bus, charter jet (http://www.fla-keys.com/gettinghere.cfm). For some reason they do not include public transportation options from the major airports in South Florida. The Green alternative is $4.35 from Islamorada to Miami International. It also supports the local economy and transportation system.

One route has four changes, but, they are all easy and no walking involved. The trip is a North bound trip from Florida Keys Diver Center (Flakys) to Miami Airport (MIA). To start you look out Flayks front door MM 90.5 and you will see a bench on Rt. 1 this is the bus stop for the Monroe-Dade shuttle. This over highway coach style bus will take you North (or South to Marathon) to Florida City terminating at Florida City Walmart. The trip fare is $1.85 exact change required. The Dade-Monroe Express #301 has Seventeen departure times from Florida City to Key Largo starting at 5:15am and stoping at 10:00pm. The North bound route has Seventeen departure times as well starting at 7:35am and stoping at 12:25 am.
http://www.monroecounty-fl.gov/Pages/.... You usually have a little time to get a coffee at McD's or to grab a few items in Walmart. From here (same location you are dropped off at) you board the # 38 bus heading to the metro rail system(http://www.co.miami-dade.fl.us/transit/). The fare is $2.00 and be sure to get a rail transfer. The # 38 takes you on the bus way for a good distance to the South Dadeland metro rail station. Here you board the train (only goes north last stop) to Douglas station 5 stops up. You have to get a rail to bus transfer for $ .50 before you get on the metro rail (http://www.co.miami-dade.fl.us/transi...). The final transfer is onto the J bus (http://www.co.miami-dade.fl.us/transit/) at Douglas station this takes you to Miami Airport and the Tri-Rail to Fort Lauderdale ($3.75) or West Palm Beach ($6.90) (http://www.tri-rail.com/). The trip between Florida Keys Dive Center and Miami International Airport takes about four hours compared to about two with a car. Some may argue on that but rental counters and drop offs take time and for the traveling diver  this route may add a couple of hours of nap time for a lot less money.

The route: Monroe-Dade shuttle connect to Miami Dade bus 38 connect to Metro rail connect to Miami Dade J bus. Heading south do the reciprocal.