Preliminary figure for operating profit was 75.3 billion won (67 million U.S. dollars) in the October-December quarter, down 79.5 percent from a year earlier. It far missed market expectations of about 398 billion won (353 million U.S. dollars).
Revenue declined 7 percent over the year to 15.8 trillion won (14.1 billion U.S. dollars) in the December quarter.
Net income and earnings of each business would be announced later this month after external audit.
Despite the slumping fourth-quarter earnings, the South Korean company's operating profit for the whole year of 2018 reached a new high of 2.7 trillion won (2.4 billion U.S. dollars). The previous high was tallied in 2009.
The 2018 revenue amounted to 61.3 trillion won (54.5 billion U.S. dollars), the second-largest ever recorded by the tech firm.
It was attributable to strong sales of premium TVs and home appliances such as washing machine and refrigerators.
LG's mobile business, however, was estimated to have recorded a deficit in the fourth quarter on stiffer competition with rival companies. The mobile unit stayed in the red since the second quarter of 2017.