The General Assembly first convened in Chillicothe, then the Ohio capital, on March 1, 1803.
The second constitution of Ohio, effective in 1851, took away the power of the General Assembly to only choose the state's executive officers, granting that right to the voters. A complicated formula apportioned legislators to Ohio counties and the number of seats in the legislative houses varied from year-to-year.
The Ohio Politics Almanac by Michael F. Curtin (Kent State University Press) described apportionment thus:
"The new  constitution ... contained a complicated formula for apportionment, the so-called "major fraction rule." Under it, the state's population was divided by 100, with the resulting quotient being the ratio of representation in the House of Representatives. Any county with a population equal to at least half the ratio was entitled to one representative; a county with a population of less than half the ratio was grouped with an adjacent county for districting; a county containing a population of at least one and three-fourths the ratio was entitled to two representatives; a county with a population equal to three times the ratio was entitled to three representatives. To determine Senate districts, a similar procedure was followed; the starting point, however was figured by dividing the state's population by 35. The ratios for the House and Senate and the resulting apportionment was determined by a board consisting of the governor, auditor, and secretary of state."
In 1923, the apportionment system was modified by the Hanna amendment, which also gave the governor veto power over the assembly's acts, which could be overridden by a two-thirds vote of the assembly's houses. The last state constitutional convention, held in 1912, gave the governor a line-item veto, but reduced the supermajority required for overriding the veto to three-fifths. In 1956, a referendum increased the terms of state senators from two to four years.
The Hanna amendment (which guaranteed each county at least one representative and all members elected at large) guaranteed that rural areas of Ohio would dominate the legislature by giving them disproportionate representation. Several decisions by the U.S. Supreme Court in the 1960s, however, mandated apportionment proportional to population. Reapportionment was ordered in 1964. Starting with the 1966 election, the number of seats in the two chambers were fixed at their present numbers of 33 and 99.
Republican activists, led by Fred A. Lennon, began pursuing term limits in the 1980s, in 1992, a referendum set term limits of eight consecutive years—four consecutive terms in the house and two consecutive terms in the senate. Terms are considered consecutive if they are separated by less than four years. Once serving the limit former members are re-eligible for election to the legislature after four years.