As of the census of 2000, there were 750,162 people, 265,815 households, and 190,855 families residing in the county. The population density was 2,422 people per square mile (935/km²). There were 273,637 housing units at an average density of 884 per square mile (341/km²). The racial makeup of the county was 68.42% White, 9.13% Black or African American, 0.20% Native American, 13.89% Asian, 0.04% Pacific Islander, 5.71% from other races, and 2.60% from two or more races. 13.59% of the population were Hispanic or Latino of any race. 15.0% were of Italian, 9.8% Irish, 8.0% Polish and 6.2% German ancestry according to Census 2000.
There were 265,815 households out of which 34.20% had children under the age of 18 living with them, 57.00% were married couples living together, 10.80% had a female householder with no husband present, and 28.20% were non-families. 22.40% of all households were made up of individuals and 8.70% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.23.
In the county the population was spread out with 23.70% under the age of 18, 9.50% from 18 to 24, 32.80% from 25 to 44, 21.70% from 45 to 64, and 12.30% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 96.40 males. For every 100 females age 18 and over, there were 93.50 males.
The median income for a household in the county was $61,446, and the median income for a family was $70,749 (these figures had risen to $74,732 and $86,239 respectively as of a 2007 estimate). Males had a median income of $49,683 versus $35,054 for females. The per capita income for the county was $26,535. About 4.20% of families and 6.60% of the population were below the poverty line, including 7.20% of those under age 18 and 6.00% of those age 65 or over.
Middlesex County hosts various state routes, US Routes, Interstates and toll highways. The state routes are: Route 18, Route 26, Route 27, Route 28, Route 32, Route 33 (only in Monroe), Route 34 (only in Old Bridge), Route 35, Route 91, Route 171 and Route 440.
US Routes include: U.S. Route 1, U.S. Route 9, U.S. Route 1/9, and U.S. Route 130.
Middlesex hosts a few highways/interstates as well. Middlesex County hosts the southern end of the Middlesex Freeway (Interstate 287) which then turns into the Middlesex Frwy Ext. (NJ 440), which connects to the Outerbridge Crossing. The Garden State Parkway passes through the eastern edge of the county, which features the northern start/end of the split-roadways (Express & Local Lanes). The New Jersey Turnpike carries Interstate 95 through the county. The Turnpike hosts the southern start/end of the "dual-dual" configuration (inner car lanes and outer truck lanes) in Cranbury Township/Monroe Township.
The NJDOT is currently upgrading the Route 18 "avenue" to a freeway between the Route 1 interchange all the way up to the new 18 Extension in Piscataway Township. The NJTPA planned to build Route 92, which was to start near the intersection of Ridge Road & Route 1 in South Brunswick Township to Interchange 8A in Monroe Township. This plan was cancelled on December 1, 2006. Instead, the TPA will extend the "dual-dual" from Monroe Township, south to the interchange with the Pennsylvania Extension (Exit 6) in Mansfield Township.
Middlesex County is served by New Jersey Transit for rail service and both New Jersey Transit and Coach USA for bus service. There are bus routes that serve all townships in the county. The main rail lines that serve Middlesex are: North Jersey Coast Line, Northeast Corridor Line, and Raritan Valley Line. The North Jersey Coast Line runs through the eastern part of the county. The Northeast Corridor Line runs through the northern and central part of the county. The Raritan Valley Line serves some communities along the county's northern border with Union County.
Intercity rail service is also provided by Amtrak. The routes that runs through Middlesex are Acela Express, Keystone, Regional, and Vermonter services.